Respo – Histo

✅ Correct Answer: The presence of cartilage

Explanation:
The presence or absence of cartilage is the key histological feature that distinguishes bronchi from bronchioles under the microscope.

• Bronchi contain irregular plates of hyaline cartilage in their walls, which help keep the airways open.

• Bronchioles, on the other hand, completely lack cartilage — instead, their walls are supported by smooth muscle that controls airway diameter through contraction and relaxation.

This transition marks the shift from the conducting zone with structural support (bronchi) to the smaller, more flexible airways (bronchioles).

❌ The presence of smooth muscle
Both bronchi and bronchioles contain smooth muscle, so it does not distinguish them.

❌ The type of epithelium
Both may have ciliated columnar or cuboidal epithelium, depending on size; epithelial type gradually changes but is not the defining feature.

❌ The type of cartilage
Only bronchi have cartilage, so “type” is irrelevant for bronchioles (they have none).

❌ The absence of glands
While bronchi have submucosal glands, and bronchioles generally lack them, this feature is secondary — the absence of cartilage is the most distinctive and reliable difference.

✅ Correct Answer: Great alveolar cells

Explanation:
Great alveolar cells, also known as Type II alveolar cells (Type II pneumocytes), are cuboidal epithelial cells found within the alveolar walls.
Their key features and functions include:

• Cuboidal shape — distinct from the thin, squamous Type I cells.

• Secretion of pulmonary surfactant, a phospholipid-rich substance that reduces surface tension and prevents alveolar collapse.

• Ability to divide and regenerate both Type I and Type II pneumocytes after lung injury, making them essential for alveolar repair.

Thus, the cuboidal shape and surfactant secretion are the hallmark features that identify these as Great (Type II) alveolar cells.

❌ Squamous alveolar cells
These are flattened cells — not cuboidal — and form the respiratory surface for gas exchange (Type I pneumocytes).

❌ Type 1 alveolar cells
Same as squamous alveolar cells — thin and flat, covering ~95% of alveolar surface area; do not secrete surfactant.

❌ Alveolar atria
These are spaces or passageways leading into groups of alveoli, not cellular structures.

❌ Alveolar sac
A cluster of alveoli that share a common opening — again, a structural space, not a cell type.

✅ Correct Answer: The interrupted wall of lumen

Explanation:
The respiratory bronchioles are the first part of the respiratory zone of the lung — the region where gas exchange begins.
They are distinguished histologically by having an interrupted (discontinuous) wall, where alveoli open directly into the bronchiolar lumen.

Key features:

• Lined partly by ciliated cuboidal epithelium and partly by squamous alveolar cells.

• No cartilage or submucosal glands.

• Serve as a transition between the terminal bronchioles (conducting zone) and alveolar ducts (respiratory zone).

Thus, the presence of alveolar outpouchings (interruptions) in their walls is the hallmark feature that differentiates respiratory bronchioles from other bronchioles.

❌ The type of epithelium
Although respiratory bronchioles have a mix of ciliated cuboidal and alveolar epithelium, this overlap is not the key identifying feature — the interrupted wall is.

❌ The presence of glands
Bronchioles, including respiratory ones, lack submucosal glands — glands are found only in larger bronchi.

❌ The diameter of lumen
The lumen gradually narrows along the airway tree, but size alone does not define the respiratory bronchiole.

❌ The number of cells
Cell count is variable and non-defining; structure and wall composition are more diagnostic.

✅ Correct Answer: Type 2 alveolar epithelial cells

Explanation:
Type II alveolar epithelial cells (also called Type II pneumocytes) are specialized cuboidal cells located within the alveolar walls.
Their main function is to synthesize and secrete pulmonary surfactant, a lipoprotein-rich substance composed primarily of dipalmitoylphosphatidylcholine (DPPC).

Surfactant reduces alveolar surface tension, preventing alveolar collapse (atelectasis) during exhalation and ensuring efficient gas exchange.
Type II cells also serve as progenitor cells, capable of dividing and differentiating into Type I pneumocytes after lung injury.

❌ Squamous epithelial cells of trachea
These are protective lining cells, not involved in surfactant secretion; the trachea is lined by pseudostratified ciliated columnar epithelium.

❌ Type 1 alveolar epithelial cells
These are thin, squamous cells that form the respiratory surface for gas exchange, not surfactant secretion.

❌ Endothelial cells
Line capillaries and facilitate gas diffusion; they play no role in surfactant production.

❌ Secretory cells
A general term; while many tissues have secretory cells, surfactant is specifically secreted by Type II pneumocytes.

✅ Correct Answer: Stratified squamous non-keratinized

Explanation:
The vocal cords (true vocal cords) are lined by stratified squamous non-keratinized epithelium, which is well-suited for protection against mechanical stress caused by vibration during phonation.

• This type of epithelium provides durability while maintaining flexibility and moisture, essential for sound production.

• In contrast, most of the laryngeal mucosa (including the false vocal cords) is lined by pseudostratified ciliated columnar epithelium (respiratory type).

This change in epithelial type at the true vocal cords is a classic example of functional adaptation — a tougher epithelium where friction and trauma are greatest.

❌ Stratified squamous keratinized
Found in skin and lips, where protection from drying and abrasion is needed; not suitable for the moist internal environment of the larynx.

❌ Stratified cuboidal
Seen in ducts of sweat and salivary glands, not in respiratory or phonatory structures.

❌ Columnar ciliated
Lines most of the respiratory tract, including trachea and bronchi, but not the vocal cords themselves.

❌ Simple squamous
Present in alveoli and endothelium, specialized for gas exchange, not for friction resistance.

The trachea is kept open — or maintains its patency — by the presence of C-shaped hyaline cartilaginous rings in its wall.
These rings are incomplete posteriorly, where the trachealis muscle bridges the gap adjacent to the esophagus. The cartilage prevents the airway from collapsing during inspiration when intrapulmonary pressure becomes negative, ensuring a steady airflow to and from the lungs.

The posterior membranous part allows slight compression of the trachea during swallowing while maintaining airway stability — a fine balance between rigidity and flexibility.

❌ Why the Other Options Are Incorrect

• Pseudostratified ciliated epithelium:
  Lines the trachea and helps trap and move mucus upward toward the pharynx (mucociliary escalator), but it doesn’t provide structural support.

• Surface tension of water:
  Actually collapses air spaces (like alveoli), not maintains patency — it’s something the lungs must overcome.

• Air pressure:
  Intraluminal air pressure fluctuates during breathing but isn’t sufficient alone to keep the trachea open without cartilage support.

• Surfactant:
  Reduces surface tension in alveoli, preventing alveolar collapse — it acts at the microscopic lung level, not the trachea.

✅ Bipolar — Correct.
Olfactory neurons are bipolar neurons, each having a single dendrite projecting to the surface with olfactory cilia and a single axon that passes through the cribriform plate to synapse in the olfactory bulb. This structure allows them to detect odor molecules and transmit signals directly to the brain.

❌ Multipolar — Found in motor neurons, not sensory olfactory cells.
❌ Pseudounipolar — Typical of sensory neurons in dorsal root ganglia, not in the olfactory epithelium.
❌ Pseudo bipolar — Not a recognized neuronal classification.
❌ Unipolar — Rare in humans; not characteristic of olfactory receptors.

✅ Correct answer: Distal part of alveolar duct

Let’s go through this carefully 👇

Detailed Explanation:

• Smooth muscle progressively decreases as airways branch deeper.
• It is prominent in bronchi, thin in bronchioles, and fragmented as small muscle knobs in respiratory bronchioles and alveolar ducts.
• These muscle fibers disappear completely only beyond the distal part of the alveolar ducts, i.e., in the alveolar sacs and alveoli themselves.

So, the last level where smooth muscle is still present (though sparse) is the distal part of the alveolar duct.

Why the others are incorrect:

• Present throughout the respiratory system → ❌ Wrong; smooth muscle ends before the alveoli.
• Terminal bronchioles → ❌ Smooth muscle is still well developed here.
• Alveoli → ❌ No smooth muscle fibers here at all.
• Bronchi → ❌ Have thick smooth muscle layers, much higher up in the airway tree.

Extrapulmonary Bronchi (Right and Left Main Bronchi):

• Structurally and histologically similar to the trachea because they are outside the lungs.

• They have:

  • Pseudostratified ciliated columnar epithelium

  • Seromucous glands in the submucosa

  • C-shaped hyaline cartilage rings

• As they enter the lungs, their structure transitions to intrapulmonary bronchi, where the cartilage becomes irregular plates instead of rings.

Hence, the extrapulmonary bronchus has the same histological appearance as the trachea.

❌ Why the Other Options Are Incorrect:

• Intrapulmonary bronchus:
  Cartilage is not C-shaped, but present as discontinuous plates.
  Surrounded by lung tissue, and the wall is thinner than the trachea’s.

• Pre-terminal bronchioles:
  Lack cartilage and submucosal glands.
  Epithelium transitions to simple ciliated columnar.

• Terminal bronchioles:
  Smallest part of the conducting system; no cartilage, no glands, no goblet cells.
  Epithelium: Simple cuboidal ciliated with Clara (club) cells.

• Respiratory bronchioles:
  Belong to the respiratory zone, having alveoli in their walls for gas exchange.
  Epithelium: Simple cuboidal non-ciliated in alveolar regions.

Respiratory bronchioles mark the transition from the conducting portion to the respiratory portion of the airway — they are the first part of the respiratory zone where gas exchange begins.

They arise from terminal bronchioles and are characterized by:

• Walls interrupted by alveoli, which allows for limited gas exchange.

• Absence of cartilage and glands.

• Presence of Clara cells (now called Club cells) — non-ciliated, dome-shaped cells that secrete surfactant-like material, detoxify harmful substances, and regenerate bronchiolar epithelium.

Hence, it is incorrect to say that respiratory bronchioles lack Clara cells — they are, in fact, a defining feature of these airways.

❌ Why the Other Options Are Incorrect

• Have an interruption of alveoli in its walls:
  ✅ Correct — this is the hallmark of respiratory bronchioles; some portions of their wall open into alveoli.

• First part of the respiratory portion:
  ✅ Correct — they represent the beginning of the gas exchange zone.

• Lack glands in its walls:
  ✅ Correct — glands are absent beyond the larger bronchi.

• Lack goblet cells:
  ✅ Correct — goblet cells disappear beyond the terminal bronchioles; mucus would obstruct gas exchange here.

Distinguishing features between bronchi and bronchioles include:

✅ Key point: Both bronchi and bronchioles contain smooth muscle, so this is not a distinguishing feature.

Elastic Fibers in the Distal Respiratory Tract

1. Alveolar ducts:

   • Surrounded by smooth muscle and elastic fibers, leading into alveolar sacs

   • Elastic fibers support recoil of ducts and adjacent alveoli

2. Alveolar sacs:

   • Composed of clusters of alveoli, with thin walls

   • Contain elastic fibers, but the density is less than in alveolar ducts, which serve as the main elastic “spring” for recoil

3. Interalveolar septa:

   • Contain capillaries and some elastic fibers

   • Main function is gas exchange, not elastic recoil

4. Respiratory bronchioles:

   • Mostly smooth muscle and epithelium, fewer elastic fibers

✅ Conclusion:

• The alveolar duct contains the greatest number of elastic fibers, more than the alveolar sac.

• Alveolar sacs rely on the fibers in alveolar ducts and septa but themselves have less dense elastic tissue.

The trachealis muscle is a band of smooth muscle that bridges the posterior gap between the C-shaped hyaline cartilage rings of the trachea. It lies posterior to the trachea, adjacent to the esophagus, allowing flexibility during swallowing.

🔹 Structure and Function:

• The trachea is composed of 16–20 C-shaped hyaline cartilage rings that keep the airway open.

• The open posterior ends of these rings are connected by the trachealis muscle and fibroelastic tissue.

• The trachealis contracts to narrow the tracheal lumen, increasing airflow velocity during coughing, which helps expel mucus or foreign particles.

• It also relaxes during swallowing, allowing the esophagus (situated just behind the trachea) to expand anteriorly.

Since its actions are involuntary and slow, the trachealis muscle is composed of smooth muscle fibers, controlled by the autonomic nervous system (particularly the parasympathetic fibers from the vagus nerve, which cause contraction).

❌ Why the Other Options Are Incorrect:

• Skeletal and smooth:
  The trachealis consists solely of smooth muscle fibers — there’s no mixture of skeletal muscle here.

• Skeletal:
  Skeletal muscle is voluntary and striated, found in structures like the tongue or pharynx, not in the trachea.

• Cardiac:
  Found only in the heart; characterized by intercalated discs and involuntary rhythmic contraction — unrelated to the trachea.

• Hyaline:
  Refers to the cartilage forming the C-shaped rings of the trachea, not the muscle connecting their ends.

The bronchioles are the smallest conducting airways before the respiratory portion of the lung begins.
They are characterized by:

• Absence of cartilage (unlike bronchi)

• Abundance of smooth muscle in their walls

• Ability to constrict or dilate to control airflow resistance

Because of this, bronchioles have the maximum proportion of smooth muscle relative to their wall thickness.

❌ Why the Other Options Are Incorrect

• Trachea: Has C-shaped hyaline cartilage rings and very little smooth muscle (only in the trachealis at the posterior wall).

• Bronchi: Contain cartilage plates and some smooth muscle, but not as much as bronchioles.

• Respiratory bronchioles: Mark the transition to the respiratory zone — they have few scattered smooth muscle fibers.

• Alveoli: Have no smooth muscle, only elastic fibers and capillary networks for gas exchange.

Clara cells (Club cells) are non-ciliated, secretory epithelial cells found in the bronchioles. They:

• Secrete a component of surfactant (mainly phospholipids and proteins) that reduces surface tension in small airways, helping prevent their collapse during expiration

• Act as progenitor cells, replacing damaged bronchiolar epithelium

Other options:

• Engulfing microorganisms → function of alveolar macrophages

• Ciliary activity → ciliated epithelial cells, not Clara cells

• Producing mucus → goblet cells

The olfactory epithelium is a specialized region responsible for smell, located in the roof of the nasal cavity. It also extends slightly onto:

• Upper part of the nasal septum (medial wall)

• Superior nasal concha (lateral wall)

It contains:

• Olfactory receptor cells

• Supporting (sustentacular) cells

• Basal cells (progenitors for new receptor cells)

Other options:

• Lateral walls / Medial walls / Floor → mainly lined by respiratory epithelium.

• Pharynx → not involved in olfaction.

The trachea and esophagus can be distinguished histologically by their epithelial lining:

• Trachea → ciliated pseudostratified columnar epithelium with goblet cells (respiratory epithelium)

• Esophagus → stratified squamous non-keratinized epithelium

Other layers (blood vessels, adventitia, submucosa, glands) are present in both organs and are not distinctive enough for differentiation.

The interalveolar septum is the thin wall between adjacent alveoli, primarily designed for gas exchange. It contains:

• Blood capillaries – for oxygen and carbon dioxide exchange

• Reticular fibers – provide structural support

• Elastic fibers – allow recoil during expiration

• Dust cells (alveolar macrophages) – phagocytose inhaled particles

Neutrophils, however, are not normally present in the septum; they appear only during acute inflammation or infection.

Type II pneumocytes (septal cells) are cuboidal epithelial cells found in the alveoli.
They produce pulmonary surfactant, a lipoprotein substance (mainly dipalmitoyl phosphatidylcholine) that reduces surface tension, thereby preventing alveolar collapse (atelectasis) during expiration.

Other options:

• Type I pneumocytes → form the thin gas-exchange surface, not surfactant.

• Alveolar macrophages → clear debris and microbes, not surfactant producers.

• Clara (Club) cells → secrete components of surfactant in bronchioles, not alveoli.

• Epithelial cells → too general; specifically Type II cells are responsible.

Type I pneumocytes (flat, squamous cells covering ~95% of alveolar surface) are derived from Type II pneumocytes, which act as progenitor (stem-like) cells.
After alveolar injury, Type II cells divide and differentiate into Type I cells to restore the epithelium.

Other options:

• Capillary endothelium → forms capillary lining, not alveolar epithelium.

• Alveolar macrophages → derived from monocytes, not epithelial origin.

• Brush border cells → found in bronchioles, not alveoli.

• Epithelial cells → too general; both pneumocytes are epithelial, but specifically Type II gives rise to Type I.

The olfactory epithelium is located in the roof of the nasal cavity, extending slightly onto the upper part of the nasal septum (medial wall) and superior nasal concha (lateral wall).
It contains olfactory receptor cells, supporting (sustentacular) cells, and basal cells responsible for smell perception.

Other options:

• Lateral wall / Medial wall / Floor → may contain respiratory epithelium, not olfactory.

• Vibrissae → coarse hairs in the vestibule, not part of olfaction.

Immotile Cilia Syndrome, also known as Kartagener’s syndrome (a type of Primary Ciliary Dyskinesia), is caused by a defect in the dynein arms of cilia and flagella.

🔬 Normal Ciliary Structure (Axoneme):

Cilia and flagella share a common internal structure called the axoneme, with the “9 + 2” arrangement:

• 9 peripheral doublets of microtubules

• 2 central single microtubules

• Connected by nexin links and powered by dynein arms

🧩 Dynein arms (both inner and outer) are ATPase motor proteins that generate sliding forces between adjacent microtubule doublets → this sliding is converted into bending motion of the cilium.

⚕️ Defect in Dynein Arms → Immotile Cilia:

When dynein arms are absent or dysfunctional:

• Cilia cannot beat properly → impaired mucus clearance → chronic respiratory infections, sinusitis, bronchiectasis

• Sperm flagella become immotile → male infertility

• Random organ rotation during development → situs inversus in about 50% of cases

⚖️ Why Other Options Are Incorrect:

The respiratory epithelium (lining the nasal cavity, trachea, and bronchi) is a pseudostratified ciliated columnar epithelium with goblet cells — a classic structure for mucociliary clearance.

🔬 Cell Types in Respiratory Epithelium:

🚦 Mechanism:

The beating cilia on columnar cells move the mucus blanket upward toward the pharynx — this is the mucociliary escalator, a key defense mechanism of the respiratory tract.

Damage to these cilia (e.g. in smokers or primary ciliary dyskinesia) impairs mucus clearance, predisposing to infections.

The trachea is a fibro-cartilaginous, mucous-membrane-lined tube connecting the larynx to the bronchi.
Histologically, it has four main layers (from inside outward):

1. Mucosa – pseudostratified ciliated columnar epithelium with goblet cells

2. Submucosa – contains seromucous glands

3. Cartilaginous layer – C-shaped hyaline cartilage rings joined posteriorly by trachealis muscle

4. Adventitia – outer connective tissue that blends with surrounding structures

⚙️ Key Structural Facts:

So, the odd statement is that the cartilage is “in the adventitia” — it is not.
The C-shaped cartilage lies deep to the submucosa and superficial to the adventitia.

⚖️ Why Others Are Correct:

• Fibroelastic membrane in lamina propria: ✅ True.

• Epithelium columnar ciliated: ✅ True (pseudostratified).

• Thick basement membrane: ✅ Characteristic of trachea.

• Seromucous glands in submucosa: ✅ True.

The intra-alveolar septa are the thin walls that separate adjacent alveoli. They contain:

• Capillaries for gas exchange

• Connective tissue fibers (elastic and reticular)

• Cells such as type I and type II pneumocytes, fibroblasts, and macrophages (dust cells)

Dust cells are alveolar macrophages — phagocytic cells derived from monocytes. They play a key role in:

• Phagocytosing dust particles, pathogens, and debris

• Migrating to the bronchioles or lymphatic system to remove engulfed material

• Protecting alveolar surfaces from infection and inflammation

❌ Why other options are incorrect

• Sustentacular cells: Supportive cells found in olfactory epithelium, not in the alveoli.

• Columnar cells: Present in the respiratory epithelium of bronchi and bronchioles, not within the alveolar septa.

• Receptor cells: Found in the olfactory mucosa, responsible for smell detection — absent in lungs.

• Basal cells: Stem cells located in pseudostratified ciliated epithelium of the conducting airways, not in alveoli.

✅ Correct Answer: Type 2 alveolar epithelial cells

Explanation:
Type II alveolar epithelial cells (also called Type II pneumocytes) are specialized cuboidal cells located in the alveolar walls, usually at the corners of the alveoli. Their main function is to secrete pulmonary surfactant, a lipoprotein substance that reduces surface tension within the alveoli and prevents their collapse (atelectasis) during exhalation.
Surfactant also helps maintain alveolar stability, improves lung compliance, and plays a role in host defense.

❌ Squamous epithelial cells of trachea:
These cells line the upper respiratory tract and serve a protective role. They do not participate in gas exchange or surfactant secretion.

❌ Type 1 alveolar epithelial cells:
These are flat squamous cells that cover about 95% of the alveolar surface area. Their function is gas exchange, not secretion. They form part of the blood-air barrier, allowing oxygen and carbon dioxide to diffuse efficiently.

❌ Endothelial cells:
These line the capillaries within the alveolar septa and are involved in gas exchange and fluid balance, not surfactant production.

❌ Secretory cells:
This is a nonspecific term; various secretory cells exist in the respiratory tract (like Clara cells in bronchioles), but not in the alveoli for surfactant secretion. The specific surfactant-producing cells are Type II pneumocytes.

✅ Correct Answer: Inter-alveolar septum

Explanation:
The inter-alveolar septum is the thin wall that separates adjacent alveoli. It contains the capillary network through which gaseous exchange occurs between the air in the alveoli and the blood. Structurally, it consists of:

• Two layers of alveolar epithelium (simple squamous epithelium made of type I pneumocytes)

• A fused basement membrane between them

• Interstitial tissue containing elastic fibers, collagen fibers, and capillaries

This design provides maximum surface area with minimal diffusion distance, facilitating efficient gas exchange.

❌ Alveolar epithelium:
This refers to the lining of an individual alveolus, composed of type I and type II pneumocytes. It forms part of the septum but does not lie between two alveoli.

❌ Alveolar phagocytes:
Also called dust cells, these are macrophages that remove debris and microorganisms within the alveolar lumen. They do not form a structural barrier between alveoli.

❌ Alveolar ducts:
These are passages that connect the respiratory bronchioles to the alveolar sacs. They lead toward alveoli, not between them.

❌ Atrium:
The alveolar atrium is the central space into which multiple alveolar ducts open. It does not lie between alveoli; rather, it serves as a common opening area.

The olfactory epithelium is a specialized region of the nasal mucosa located in the roof of the nasal cavity, along the superior nasal concha and part of the nasal septum.

It is a pseudostratified columnar epithelium with glandular cells and consists of three major cell types:

1. Olfactory receptor cells – These are bipolar neurons whose dendrites bear olfactory cilia that detect odorant molecules. Their axons pass through the cribriform plate to synapse in the olfactory bulb.

2. Supporting (sustentacular) cells – Columnar cells that provide mechanical and metabolic support to receptor neurons.

3. Basal cells – Small regenerative cells at the base of the epithelium that replace worn-out olfactory neurons and supporting cells.

Additionally, beneath the epithelium are Bowman’s (olfactory) glands, which are glandular structures that secrete a watery fluid to dissolve odorants, facilitating their detection.

❌ Why Other Options Are Incorrect:

• Pseudostratified columnar without glandular cells: Incorrect because Bowman’s glands are an essential feature of the olfactory mucosa.

• Stratified squamous: Found in regions of the nasal cavity exposed to friction, such as the vestibule, but not in the olfactory area.

• Simple cuboidal: Not found in the nasal cavity; typically lines ducts and small glandular passages.

• Simple columnar: Too simple for the complex sensory function of the olfactory epithelium.

✅ Correct Answer: Clara cells

Explanation:

Clara cells (also known as club cells) are non-ciliated, dome-shaped secretory cells found only in the bronchioles of the lower respiratory tract. Their main functions include detoxification, secretion of surfactant-like material, and regeneration of bronchiolar epithelium.

They are not present in the mucosal lining of the nasopharynx, which is part of the upper respiratory tract.

The nasopharynx is lined by pseudostratified ciliated columnar epithelium with goblet cells, specialized for filtering, warming, and humidifying inspired air — not for the detoxification or repair functions of Clara cells.

❌ Diffuse neuroendocrine cells (DNES):

These are present throughout the respiratory tract, including the nasopharynx, and play a role in local regulation via neuropeptides.

❌ Ciliated columnar cells:

These are the main cells of the respiratory epithelium in the nasopharynx, responsible for moving mucus and trapped particles toward the pharynx.

❌ Goblet cells:

Present in the nasopharynx, they secrete mucus that traps dust and microorganisms.

❌ Serous cells:

Found in the submucosal glands of the nasopharynx; they secrete watery fluid with enzymes that help moisten and protect the mucosa.

The respiratory tract progressively changes its histological structure as it moves from the trachea down to the alveoli. One of the most important transitions is the decrease in cartilage and the increase in smooth muscle until the terminal bronchioles.

Let’s look at the pattern along the airway:

1. Trachea – Has C-shaped cartilage rings with little smooth muscle (only the trachealis muscle at the posterior wall).

2. Bronchi – Have irregular cartilage plates and moderate smooth muscle.

3. Bronchioles – No cartilage at all, but abundant circular smooth muscle in the wall. This smooth muscle regulates the airway diameter and thus airflow resistance.

Therefore, among all the listed options, the bronchioles contain the greatest relative amount of smooth muscle, as they rely entirely on it to control airflow and prevent collapse.

❌ Why Other Options Are Incorrect:

• Nasal cavity:
  Lined by respiratory epithelium with a lamina propria, but no smooth muscle in the wall; its support comes from bone and cartilage.

• Trachea:
  Contains C-shaped cartilage rings and only a thin trachealis muscle posteriorly; not rich in smooth muscle.

• Bronchi:
  Contain cartilage plates and some smooth muscle, but the smooth muscle proportion is less than in bronchioles.

• Larynx:
  Contains skeletal muscles for phonation and control of the vocal cords, not smooth muscle.

The interalveolar septum (alveolar wall) separates adjacent alveoli and is the primary site of gas exchange. It is composed of:

• Type I pneumocytes – thin squamous cells for diffusion

• Type II pneumocytes – surfactant-secreting cuboidal cells

• Capillaries – where gas exchange occurs

• Alveolar macrophages (dust cells) – immune cells that remove inhaled particles and debris

Dust cells can be found within the septum or on the alveolar surface. They migrate toward the bronchioles, carrying foreign matter out of the lungs.

❌ Why the Other Options Are Incorrect:

• Sustentacular cells: Found in sensory epithelia like the olfactory mucosa, not in the alveoli.

• Receptor cells: Responsible for sensory detection (e.g., smell, taste), not respiratory defense.

• Conical cells: Not a recognized histological cell type in lung tissue.

• Basal cells: Present in the pseudostratified columnar epithelium of larger airways (e.g., trachea), not within alveoli.

🏥 Clinical Correlation:
In chronic passive congestion of the lungs (e.g., left-sided heart failure), alveolar macrophages engulf hemosiderin from extravasated RBCs. These are called “heart failure cells”, and their presence in sputum indicates pulmonary venous congestion.

In the trachea, the submucosa contains mixed (seromucous) glands that help moisten inspired air and trap dust particles.
Histologically, these glands are classified as branched tubulo-alveolar glands, because they have both:

• Tubular portions — that secrete mucus

• Alveolar (acinar) portions — that secrete serous fluid

This combination allows for both lubrication and antimicrobial defense of the airway.

❌ Why the Other Options Are Incorrect:

• Simple tubulo-alveolar glands:
  Tracheal glands are branched, not simple — they open into several ducts.

• Branched alveolar glands:
  Only alveolar (acinar) structures — would not include the tubular component needed for mucus secretion.

• Branched tubular glands:
  Found in the stomach and intestines, not the trachea.

• Simple tubular glands:
  Characteristic of intestinal crypts, not respiratory glands.

In the respiratory tract, the presence or absence of cartilage and smooth muscle helps distinguish different airway levels histologically:

• Trachea: Large tube with C-shaped hyaline cartilage rings and trachealis muscle posteriorly.

• Bronchi: Smaller than trachea (about 1–10 mm in diameter), but still contain cartilage plates (not complete rings) and smooth muscle spiraling in the wall.

• Bronchioles: <1 mm in diameter, no cartilage in their wall, but still have smooth muscle.

• Alveolar ducts and sacs: No cartilage, minimal smooth muscle, and mainly composed of alveoli.

A 2 mm diameter tubule containing both smooth muscle and cartilage clearly corresponds to a bronchus.

❌ Why the Other Options Are Incorrect:

• Alveolar duct: Lined by alveoli; no cartilage or smooth muscle.

• Alveolar sac: Terminal cluster of alveoli — no supporting structures like cartilage or muscle.

• Bronchiole: Too small (≤1 mm) and lacks cartilage.

• Trachea: Much larger (>10 mm) with cartilage rings, not plates.

✅ Correct Answer:

Stratified squamous non-keratinized

The vocal cords (true vocal folds) are lined by stratified squamous non-keratinized epithelium.
This type of epithelium provides protection against mechanical stress and friction caused by the vibration of the vocal cords during phonation (speech).

Most of the rest of the larynx, including the false vocal cords and epiglottis (except at their tips), is lined by pseudostratified ciliated columnar epithelium (respiratory type).

❌ Incorrect Options:

• Stratified squamous keratinized — ❌ Found in skin and areas exposed to drying (like the oral mucosa), not in the moist laryngeal surface.

• Stratified cuboidal — ❌ Seen in ducts of sweat and salivary glands, not in the respiratory tract.

• Columnar ciliated — ❌ Found in the respiratory epithelium of the trachea and bronchi, not on the vibrating folds.

• Simple squamous — ❌ Found in areas of diffusion (like alveoli), not in the larynx.

✅ Correct Answer:

Dynein arms

Immotile cilia syndrome, also known as Kartagener’s syndrome (a form of primary ciliary dyskinesia), is caused by a defect in the dynein arms of cilia.

Dynein is a motor protein that connects adjacent microtubule doublets in the axoneme (the core structure of cilia and flagella) and generates the sliding motion needed for ciliary beating.
When dynein arms are defective, the cilia become immotile or dyskinetic, leading to:

• Impaired mucociliary clearance → chronic respiratory infections, sinusitis, bronchiectasis.

• Male infertility → due to immotile sperm flagella.

• Situs inversus (in some cases) → due to abnormal embryonic nodal cilia function.

❌ Incorrect Options:

• Central pair of microtubules — ❌ Present in the 9 + 2 arrangement but usually intact in this condition.

• Peripheral microtubules — ❌ The structure of the outer doublets remains normal; the motility problem arises from the missing dynein arms.

• Nexin link — ❌ Connects adjacent microtubule doublets and provides structural stability; not the usual defect in this syndrome.

• Axoneme — ❌ The entire structural core (9 + 2 arrangement); only a component (dynein arms) is defective here.

✅ Correct Answer:

Columnar cell

In the respiratory epithelium, also called pseudostratified ciliated columnar epithelium, the columnar cells are the ones that possess cilia on their apical surface.
These cilia beat in a coordinated manner to move mucus and trapped particles upward toward the pharynx, where they can be swallowed or expelled — a process known as the mucociliary escalator.

This mechanism plays a key role in keeping the airways clean and protecting the lungs from infection and debris.

❌ Incorrect Options:

• All of these — ❌ Not true; not all listed cells have cilia.

• Goblet cell — ❌ Secretes mucus, but lacks cilia.

• Basal cell — ❌ Serves as a stem cell, regenerating other epithelial cells; does not have cilia.

• Brush cell — ❌ Has microvilli, not cilia; involved in sensory and chemoreceptor functions.

✅ Correct Answer: Type I pneumocytes

Explanation:

Type I pneumocytes (also known as type I alveolar cells) are simple squamous epithelial cells that form the thin alveolar walls where gas exchange occurs. They cover about 95% of the alveolar surface area, allowing efficient diffusion of oxygen and carbon dioxide between the alveolar air and the pulmonary capillary blood. These cells are extremely thin to minimize the diffusion distance, forming part of the blood-air barrier along with the capillary endothelium and a shared basement membrane.

❌ Type II pneumocytes:

These are cuboidal cells that secrete surfactant, a phospholipid substance that reduces surface tension and prevents alveolar collapse. They also regenerate type I pneumocytes after injury, but they are not directly involved in gas exchange.

❌ Macrophages:

Also called alveolar macrophages or dust cells, these are immune cells that phagocytose debris, bacteria, and particles within the alveoli. They maintain sterility but do not participate in gas diffusion.

❌ Clara cells (Club cells):

Found in the terminal bronchioles, they secrete protective proteins and detoxify harmful substances, and act as progenitor cells for the bronchiolar epithelium. They are not present in alveoli and not involved in gas exchange.

❌ Dust cells:

Another name for alveolar macrophages, these cells remove dust and pathogens from the alveolar surface. Again, no role in gas exchange.

✅ Correct Answer: Extra-pulmonary bronchi

Explanation:

The extra-pulmonary bronchi (primary bronchi) have a histological structure very similar to the trachea. They are lined by pseudostratified ciliated columnar epithelium, contain submucosal glands, and are supported by cartilaginous plates embedded in the wall. They also have adventitia connecting them to surrounding tissues. The main difference from the trachea is that the cartilage in bronchi is plate-like rather than the C-shaped rings seen in the trachea.

❌ Pre-terminal bronchioles: Lack cartilage and submucosal glands; lined by simple cuboidal epithelium.

❌ Intra-pulmonary bronchi: Have similar features but the cartilage becomes irregular plates; still, extra-pulmonary bronchi more closely resemble the trachea histologically.

❌ Respiratory bronchioles: Begin the gas-exchanging portion; lined by simple cuboidal epithelium with occasional alveoli; no cartilage or glands.

❌ Terminal bronchioles: Lack cartilage and glands; lined by simple cuboidal epithelium with Clara cells; much smaller in diameter.

✅ Correct Answer: The absence of glands

Explanation:

Under the light microscope, one of the most reliable features distinguishing bronchioles from bronchi is the absence of submucosal glands in bronchioles. While bronchi have cartilage and submucosal glands in their walls to provide structural support and mucus secretion, bronchioles lack cartilage and glands, relying primarily on smooth muscle for airway tone. The absence of glands is therefore a key histological marker of bronchioles.

❌ The presence of smooth muscle: Both bronchi and bronchioles have smooth muscle, so this feature cannot distinguish them.

❌ The presence of cartilage: While bronchi have cartilage, some larger bronchioles may still have small cartilaginous remnants, making cartilage less consistently distinguishing than the absence of glands.

❌ The type of epithelium: Epithelium gradually transitions from pseudostratified columnar in bronchi to simple cuboidal in bronchioles, but overlap exists, so it is not a definitive marker.

❌ The type of cartilage: Bronchioles lack cartilage entirely, so discussing the type is irrelevant.

❌ The type of glands: Since bronchioles lack glands, the “type” of gland is not a distinguishing feature.

✅ Correct Answer: Pseudo-stratified columnar without goblet cells

Explanation:

The olfactory epithelium is a specialized pseudo-stratified columnar epithelium located in the roof of the nasal cavity, superior nasal concha, and upper part of the nasal septum. It contains olfactory receptor cells (bipolar neurons), supporting (sustentacular) cells, and basal cells — but no goblet cells.

This absence of goblet cells is key because mucus secretion in this area is handled by Bowman’s glands in the lamina propria, which keep the olfactory surface moist without obstructing odorant diffusion.

❌ Pseudo-stratified columnar with goblet cells:

This describes respiratory epithelium, which lines most of the nasal cavity and trachea. Goblet cells here secrete mucus, but these would block odorant molecules, making this inappropriate for olfaction.

❌ Tall columnar with microvilli:

This type is typically seen in intestinal epithelium or sensory cells of taste buds, not in the nasal mucosa.

❌ Simple columnar ciliated:

A simple columnar epithelium is not found in the nasal region — the olfactory epithelium is pseudo-stratified with specialized sensory cells.

❌ Simple cuboidal:

Represents ductal or glandular epithelium, not sensory or respiratory lining.

Explanation:

Dust cells, also known as alveolar macrophages, are phagocytic cells found on the inner surface of alveoli and within the interalveolar septa. Their main function is to engulf inhaled particulate matter, such as dust, smoke, and carbon particles, as well as microorganisms and debris. Because of this, they often appear darkly pigmented on histological sections, especially in individuals exposed to polluted air or smokers. After phagocytosis, these macrophages may migrate to the mucociliary escalator or interstitium for removal. Their dark granules are primarily carbon pigments, giving them their characteristic appearance.

❌ Type II pneumocytes – These are cuboidal epithelial cells that secrete surfactant and serve as progenitor cells for type I pneumocytes. They do not phagocytose particles or appear darkly pigmented.

❌ Type I pneumocytes – These are flattened squamous cells that form the gas exchange barrier. They cover about 95% of the alveolar surface area but have no phagocytic function.

❌ Epithelial cells – This is a general term that does not specify any particular cell type. In the alveoli, the epithelial cells include type I and type II pneumocytes, neither of which perform phagocytosis of dust.

❌ Squamous cells – These are flat epithelial cells, like type I pneumocytes, involved in gas exchange, not in phagocytosis or pigmentation.

✅ Correct answer: The interrupted wall of lumen

Explanation:

Respiratory bronchioles are the first part of the airway that participates in gas exchange, and they can be identified histologically by their interrupted (discontinuous) walls.

Respiratory bronchioles are uniquely identified by the fact that their walls are interrupted by outpouchings of alveoli — in other words, the airway lumen is broken up by alveolar openings so gas exchange begins right at their walls. This contrasts with the upstream terminal bronchioles, which have a continuous, intact wall and no alveolar invaginations; cartilage and glands are absent in both types, so those features don’t help distinguish them. Differences in epithelial cell type or lumen diameter are subtle and overlap between bronchiolar generations, and the absolute number of cells is not a reliable histologic marker; the presence of alveolar interruptions (discontinuous wall) is the clear, practical histologic clue that you are looking at a respiratory bronchiole.

The presence of glands is not a distinguishing feature because glands are found only in the larger airways — the trachea and bronchi — within the submucosa. Both terminal and respiratory bronchioles lack glands entirely. Therefore, gland presence cannot be used to differentiate respiratory bronchioles from other bronchioles. The type of epithelium also does not serve as a reliable marker, as there is a gradual transition from ciliated columnar in terminal bronchioles to cuboidal and then squamous in alveoli. This change is overlapping and not a sharp boundary. The diameter of the lumen decreases progressively from terminal to respiratory bronchioles, but this change is relative and depends on lung inflation or tissue orientation, making it unreliable for histological distinction. Similarly, the number of cells varies with the plane of the tissue section and is not a standardized criterion for classification.

Clara cells are non-ciliated, dome-shaped epithelial cells located mainly in the terminal and respiratory bronchioles.

Their key role is to secrete a surfactant-like material, which reduces surface tension within the bronchioles—helping to prevent airway collapse, especially during expiration.

Additionally, Clara cells perform several important functions:

• Detoxification: They contain cytochrome P450 enzymes that degrade harmful inhaled substances.
• Regeneration: They act as progenitor cells for the bronchiolar epithelium, replacing damaged ciliated or secretory cells.
• Immune protection: They secrete Clara cell secretory protein (CCSP), which has anti-inflammatory properties.

❌ Why the Other Options Are Incorrect:

• Regenerating new cells:
  While Clara cells do have regenerative capacity, this is not the main mechanism preventing lung collapse. The surfactant secretion plays the key role in maintaining airway stability.

• Inflating alveoli:
  Inflation of alveoli occurs through negative intrathoracic pressure during inspiration, not through Clara cell activity.

• Entrapping bodies:
  This refers more to macrophages or mucociliary clearance, not Clara cells.

• Forming memory:
  This is a function of the adaptive immune system (B and T lymphocytes), which is unrelated to the structural mechanics of lung collapse.

🩺 Clinical Connection:
Damage to Clara cells (e.g., due to smoking or toxic inhalants) can impair surfactant secretion and epithelial repair, leading to bronchiolar obstruction and increased susceptibility to respiratory diseases such as chronic bronchiolitis.

Type II pneumocytes (also called Type II alveolar cells) are specialized cuboidal epithelial cells found in the alveoli of the lungs.
Their primary function is to synthesize and secrete pulmonary surfactant, a phospholipoprotein-rich substance that reduces surface tension within the alveoli, preventing their collapse during expiration.

Surfactant production begins around 24–28 weeks of gestation and becomes adequate by 34 weeks, which is why premature infants are at risk for neonatal respiratory distress syndrome (NRDS) due to insufficient surfactant.

❌ Why the Other Options Are Incorrect:

• Type I pneumocytes:
  These are squamous epithelial cells that form the thin alveolar walls for gas exchange. They do not produce surfactant but facilitate diffusion of gases.

• Lymphocytes:
  White blood cells involved in immune defense, not in alveolar structure or surfactant production.

• Hepatocytes:
  Liver parenchymal cells responsible for metabolism, detoxification, and bile secretion, unrelated to the lungs.

• Macrophages:
  In the lungs, they are called alveolar macrophages (dust cells) and function in phagocytosis of debris and pathogens, not surfactant synthesis.

🩺Clinical Connection:
Surfactant deficiency leads to alveolar collapse (atelectasis), causing impaired gas exchange. Administration of exogenous surfactant or maternal corticosteroids can significantly improve outcomes in preterm infants

✅ Correct Answer:

Stratified squamous non-keratinized

The vocal cords (true vocal folds) are covered by stratified squamous non-keratinized epithelium. This specialized lining protects the delicate underlying structures from mechanical stress and vibration during phonation (sound production).

Most of the rest of the larynx is lined by pseudostratified ciliated columnar epithelium with goblet cells, but the vocal cords, epiglottis, and parts of the oropharyngeal inlet switch to non-keratinized squamous epithelium because these areas experience constant friction.

❌ Incorrect Options:

• Stratified squamous keratinized — ❌ Found in skin and areas exposed to drying or abrasion (like the oral mucosa or lips), not in the moist environment of the larynx.

• Stratified cuboidal — ❌ Typically lines ducts of sweat, mammary, and salivary glands, not respiratory structures.

• Columnar ciliated — ❌ Found in the respiratory epithelium of the trachea and bronchi, but not in the vocal cords themselves.

• Simple squamous — ❌ Found in alveoli and blood vessels (endothelium) for gas exchange, not in the larynx.

✅ Correct Answer:

Type II pneumocyte

Type II pneumocytes (also called Type II alveolar cells) are cuboidal or dome-shaped epithelial cells found in the alveoli of the lungs. Their main function is to produce and secrete pulmonary surfactant, a phospholipid-rich substance that reduces surface tension within the alveoli, preventing their collapse during exhalation.

Additionally, Type II pneumocytes can divide and differentiate into Type I pneumocytes after lung injury, playing a key role in alveolar repair and regeneration.

❌ Incorrect Options:

• Type I pneumocyte — ❌ These cells are flat and squamous, covering about 95% of the alveolar surface area. They are responsible for gas exchange, not surfactant production.

• Epithelial cell — ❌ A general term; many epithelial cells exist in the respiratory tract, but this does not specify surfactant-secreting cells.

• Goblet cell — ❌ Found in the airways, not alveoli; they secrete mucus, not surfactant.

• Clara cell (now called Club cell) — ❌ Found in the bronchioles, they secrete components of surfactant and detoxifying enzymes, but they are non-alveolar and not the main surfactant source.

✅ Correct Answer:

Macrophages plus monocytes

Dust cells, also known as alveolar macrophages, are specialized phagocytic cells found on the alveolar surface of the lungs. They originate from blood monocytes that migrate into the alveoli and differentiate into macrophages.

Their main function is to phagocytose inhaled particles, including dust, bacteria, and carbon particles (especially in smokers or people exposed to polluted air). When filled with these particles, they appear as darkly pigmented “dust cells.”

❌ Incorrect Options:

• Eosinophils and red blood cells — ❌ Eosinophils play a role in allergic reactions and parasitic infections, not dust clearance.

• Macrophages plus erythrocytes — ❌ Red blood cells don’t form part of these immune cells; however, macrophages may engulf RBCs during hemorrhage (forming heart failure cells), which are different from dust cells.

• Lymphocytes and plasma cells — ❌ These are part of the adaptive immune system and mainly function in antibody production, not phagocytosis.

• Neutrophils and silica — ❌ Silica damages macrophages, leading to fibrosis (silicosis), but it doesn’t create dust cells.

Correct Answer:
✅ Epithelium

• Explanation: The trachea and esophagus can be distinguished by their epithelial lining, especially in the upper chest (cervical and upper thoracic region):

  • Trachea: lined by pseudostratified ciliated columnar epithelium with goblet cells, designed for air conduction and mucus clearance.

  • Esophagus: lined by non-keratinized stratified squamous epithelium, adapted to resist friction from food passage.
    Thus, the epithelium is the key histological feature that differentiates them.

Incorrect Options:

❌ Adventitia

• Both trachea and esophagus have an outer adventitia; it doesn’t help distinguish them.

❌ Submucosa

• Both structures contain seromucous glands in the submucosa; not a reliable differentiating feature.

❌ Mucosa

• Both have mucosal layers, but the difference lies specifically in the type of epithelium within it.

❌ Muscle

• Both have muscle layers (trachea: smooth muscle posteriorly; esophagus: skeletal upper → smooth lower), but this difference is secondary; the epithelium provides the clearest distinction.

Correct Answer:
✅ C-shape cartilage plate in adventitia

• Explanation: The trachea does contain C-shaped hyaline cartilage rings, but they are located in the submucosa, not in the adventitia. These cartilaginous rings provide structural support and prevent tracheal collapse during inspiration. The adventitia is the outermost connective tissue layer, which anchors the trachea to surrounding tissues.

Incorrect Options (these are correct statements about tracheal structure):

❌ Epithelial lining resting on a thick basement membrane

• True. The trachea is lined by pseudostratified ciliated columnar epithelium with goblet cells resting on a distinct thick basement membrane.

❌ Fibroelastic membrane in deep membrane lamina propria

• True. The fibroelastic membrane (elastic tissue) helps maintain tracheal flexibility during breathing.

❌ Epithelium consists of columnar ciliated cells

• True. The epithelium has ciliated columnar cells, goblet cells, basal cells, and brush cells.

❌ Seromucous glands in submucosa

• True. These seromucous glands produce mucus and serous secretions that help trap and remove debris from inspired air.

✅ Correct Answer:

Epithelium

• The trachea is lined by respiratory epithelium → pseudostratified columnar ciliated epithelium with goblet cells.
• The esophagus is lined by stratified squamous non-keratinized epithelium, suited for protection against food abrasion.
• This difference in epithelial lining is the most reliable feature for distinguishing the two.

❌ Blood vessels

• Both the trachea and esophagus have a blood supply in the submucosa and adventitia.
• Blood vessels are not a distinguishing feature.

❌ Adventitia

• Both structures are surrounded by adventitia, which anchors them to neighboring structures.
• Hence, this does not help in differentiation.

❌ Submucosa

• Both have a submucosa layer containing connective tissue, vessels, and sometimes glands.
• Not unique enough to differentiate.

❌ Glands

• The trachea has seromucous glands in the submucosa.
• The esophagus also contains esophageal mucous glands, especially in the upper and lower parts.
• Since both contain glands, this is not a strict differentiating feature.

Smooth muscle is distributed along the respiratory tract but in varying amounts:

• Bronchi → Terminal bronchioles: Smooth muscle is well developed. Function: regulate airway diameter (bronchoconstriction/dilation) and control airflow distribution.

• Respiratory bronchioles → Proximal alveolar ducts: Smooth muscle becomes discontinuous and patchy. Function: provide limited control of airflow and maintain airway patency.

• Distal alveolar ducts: Smooth muscle completely disappears. At this level, the structure is optimized purely for gas exchange, where the thinnest possible wall is required to minimize diffusion distance for oxygen and carbon dioxide.

• Alveoli: Absolutely no smooth muscle, only type I and type II pneumocytes with elastic fibers for recoil.

Thus, the disappearance of smooth muscle at the distal part of the alveolar duct ensures efficient, unobstructed diffusion across the alveolar–capillary membrane.

❌ Why Other Options Are Incorrect

• Present throughout the respiratory system: Wrong — not present in alveoli or distal alveolar ducts.

• Alveoli: True that alveoli lack smooth muscle, but smooth muscle already disappears before this, at the distal alveolar duct.

• Terminal bronchioles: Rich in smooth muscle, crucial for airflow resistance regulation.

• Bronchi: Contain smooth muscle plus cartilage for airway patency.

The alveoli are extremely rich in elastic fibers, which are essential for their function. These fibers surround the alveolar walls and allow the lungs to expand during inspiration and recoil during expiration. This elastic recoil is what helps push air out of the lungs efficiently without excessive muscular effort.

• The fine elastic fiber network also maintains the structural integrity of the alveoli, preventing collapse while allowing flexibility.

• Damage to these fibers (e.g., in emphysema) leads to loss of recoil, air trapping, and obstructive lung disease.

❌ Why the other options are incorrect:

• Bronchioles: Have smooth muscle to regulate airway diameter but relatively fewer elastic fibers compared to alveoli.

• Trachea: Mainly supported by C-shaped hyaline cartilage rings and smooth muscle, not dominated by elastic fibers.

• Bronchi: Contain some elastic tissue, but their walls are reinforced by cartilage and smooth muscle, not primarily elastic fibers.

• Larynx: Contains hyaline and elastic cartilage, but not abundant elastic fibers in the connective tissue framework like in alveoli.

✅ Correct Answer:

Hyaline cartilage

🔎 Explanation

The olfactory mucosa (in the roof of the nasal cavity) is specialized for smell and contains:

• Sustentacular cells (supporting cells): Provide structural and metabolic support.
• Bipolar olfactory receptor neurons: Detect odor molecules.
• Bowman’s glands: Secrete serous fluid to dissolve odorants.
• Basal cells: Stem cells for regeneration.

Hyaline cartilage is found in the respiratory mucosa (supporting nasal septum, larynx, trachea), but not in the olfactory mucosa. ✅

❌ Why the others are wrong

• Sustentacular cells ❌ → Present; support olfactory neurons.
• Supporting cells ❌ → Another term for sustentacular cells, also present.
• Bipolar neurons ❌ → Main sensory cells of smell, definitely present.
• Bowman’s glands ❌ → Present; secrete mucus/serous fluid for odorant dissolution.

✅ Correct Answer: 

Clara cells

🔎 Explanation

• Clara cells (also called club cells) are non-ciliated, dome-shaped secretory cells found mainly in the terminal bronchioles.
• Their main functions are:
  • Secretion of Clara cell secretory protein (CCSP) → protects against oxidative stress and inflammation.
  • Secretion of components of surfactant.
  • Detoxification of inhaled substances via cytochrome P450 enzymes.
  • Serve as progenitor cells for regeneration of bronchiolar epithelium.
• Thus, they are the key defenders against irritants and toxins in the terminal bronchioles. ✅

❌ Why the others are wrong

• Ciliated columnar cells ❌ → Important for mucociliary clearance, but become sparse in terminal bronchioles.
• Type 1 pneumocytes ❌ → Flat cells for gas exchange in alveoli, not protective against irritants.
• Type 2 pneumocytes ❌ → Produce surfactant in alveoli, not bronchioles.
• Goblet cells ❌ → Secrete mucus in larger airways; absent in terminal bronchioles.

✅ Correct Answer:

Stratified squamous non-keratinized

• Most of the laryngeal mucosa is lined by respiratory epithelium (pseudostratified columnar ciliated epithelium with goblet cells).
• But the true vocal cords (vocal folds) are an exception — they are covered by stratified squamous non-keratinized epithelium.
• This is an adaptation to mechanical stress (vibration and friction during phonation).
• In people with heavy voice use (e.g., singers) or chronic irritation (smoking), this area can even undergo keratinization.

❌ Why not the others?

• Stratified squamous keratinized → ❌ Found in skin and high-friction areas (palms, soles), but not normally in vocal cords.
• Stratified cuboidal → ❌ Seen in ducts of sweat glands, not in the larynx.
• Columnar ciliated → ❌ Represents respiratory epithelium (covers much of the larynx), but not the true vocal cords.
• Simple squamous → ❌ Found in alveoli, endothelium, mesothelium — for diffusion, not in vocal folds.

✅ Correct Answer:

Columnar cell

Ciliated columnar epithelial cells are the hallmark of the respiratory epithelium (pseudostratified ciliated columnar epithelium). Their coordinated ciliary beating moves mucus (produced by goblet cells) and trapped particles upward toward the pharynx — the mucociliary escalator.

❌ Why not the others?

• All of these → ❌ Incorrect because not all listed cell types bear cilia.
• Goblet cell → ❌ Secretes mucus, does not have cilia.
• Brush cell → ❌ Has microvilli (sensory/receptor role), not cilia.
• Basal cell → ❌ Stem cells for epithelial regeneration, no cilia.

✅ Correct Answer: 

Myeloperoxidase

• Azurophilic granules (also called primary granules) are found in neutrophils.
• They contain myeloperoxidase (MPO), which plays a central role in the respiratory burst by generating hypochlorous acid (HOCl) from hydrogen peroxide and chloride → powerful bactericidal effect.
• These granules also contain defensins, acid hydrolases, and other antimicrobial enzymes, but MPO is the classic marker.

❌ Alkaline phosphatase

• Found in specific (secondary) granules, not in azurophilic granules.
• Used in lab tests (e.g., leukocyte alkaline phosphatase score).

❌ Collagenase

• Present in specific (secondary) granules of neutrophils, helps break down collagen during tissue invasion.
• Not in azurophilic granules.

❌ Hydrolase

• Lysosomal hydrolases are found in various lysosomes and granules, but in neutrophils, the key enzyme of azurophilic granules is MPO, which is more specific.

❌ Collagen

• Collagen is a structural protein of connective tissue, not an enzyme or a granule component.

Correct Answer:
✅ Type 2 alveolar epithelial cells

• Explanation: Type II pneumocytes (alveolar epithelial cells) are cuboidal cells found in the alveoli. They secrete pulmonary surfactant, a phospholipid-rich substance (mainly dipalmitoyl phosphatidylcholine) that reduces surface tension and prevents alveolar collapse at the end of expiration. They also serve as progenitor cells, capable of dividing and differentiating into Type I cells after lung injury.

Incorrect Options:

❌ Squamous epithelial cells of trachea

• These are part of the tracheal lining in abnormal/pathological states (normally pseudostratified ciliated epithelium), and they do not produce surfactant.

❌ Type 1 alveolar epithelial cells

• Flattened cells that cover ~95% of the alveolar surface area; their main role is gas exchange, not surfactant production.

❌ Endothelial cells

• Line pulmonary capillaries; their role is in gas exchange and barrier function, not surfactant secretion.

❌ Secretory cells

• Generic term, but surfactant is specifically secreted by Type II pneumocytes, not a general secretory cell population.

Correct Answer:
✅ They are columnar cells with microvilli at the apical surface

• Explanation: Brush cells are specialized columnar epithelial cells found in the respiratory tract epithelium. They are characterized by short, blunt microvilli on their apical surface, which likely function in sensory reception or chemoreception. These cells make contact with afferent nerve endings and may help detect changes in the chemical composition of the airway.

Incorrect Options:

❌ They are columnar cells with microvilli at the lateral surface

• Incorrect, because microvilli are confined to the apical surface, not the lateral sides.

❌ They are cuboidal cells with microvilli at the apical surface

• Wrong, brush cells are columnar, not cuboidal.

❌ They are squamous cells with microvilli at the apical surface

• Squamous cells are flat and thin, not the brush cell type.

❌ They are cuboidal cells with microvilli at the basal surface

• Incorrect, as microvilli are never found on the basal surface — only the apical surface.