Renal – Embryo

In females, the entire urethra develops from the pelvic part of the urogenital sinus.
During embryogenesis, the urogenital sinus divides into three regions:

1️⃣ Vesical part → forms most of the urinary bladder (except trigone).
2️⃣ Pelvic part → gives rise to the entire female urethra and lower part of the vagina.
3️⃣ Phallic part → contributes to the vestibule of the vagina in females (the external opening area).

Hence, the pelvic part is responsible for forming the tubular urethra extending from the bladder to the vestibule.

❌ Why the Other Options Are Wrong

• Tip:
  There’s no “tip” region in the urogenital sinus; this option is a distractor.

• Vesical part:
  Forms the bladder, not the urethra.

• Phallic part:
  Forms the vestibule of the vagina, not the urethra itself.

• Widest part:
  Anatomically imprecise — the sinus isn’t divided by width but by embryological regions (vesical, pelvic, phallic).

The penile (spongy) urethra develops from the phallic part of the urogenital sinus, which elongates during development as the genital tubercle grows into the penis.
Endoderm from this part of the sinus forms most of the penile urethral lining, except for the terminal glanular portion, which arises from an invagination of surface ectoderm at the tip of the glans that later joins the rest of the urethra.

❌ Why Others Are Wrong:

• Closure of urethral fold:
  Forms the ventral aspect of the penis and encloses the urethra, but does not form the urethral epithelium itself.

• Surface ectoderm:
  Forms only the distal glanular urethra, not the entire penile urethra.

• Pelvic part of urogenital sinus:
  Forms the prostatic and membranous urethra, not the penile portion.

• Allantois:
  Forms the urachus, which becomes the median umbilical ligament, not the urethra.

The trigone of the urinary bladder — the smooth triangular area between the two ureteric openings and the internal urethral orifice — develops from the absorbed caudal ends of the mesonephric ducts (Wolffian ducts).
Initially, the ureters open into the mesonephric ducts, but as development progresses, the lower parts of these ducts are absorbed into the posterior wall of the bladder, forming the trigone.
Although mesodermal in origin, it later becomes endodermally lined as the bladder mucosa overgrows it.

❌ Why Others Are Wrong:

• Pelvic part of urogenital sinus:
  Forms the prostatic and membranous urethra in males and the entire urethra in females, not the trigone.

• Para mesonephric duct (Müllerian duct):
  Forms female genital structures (uterus, fallopian tubes, upper vagina).

• Phallic part of urogenital sinus:
  Contributes to the penile urethra in males and vestibule in females.

• Vesical part of urogenital sinus:
  Gives rise to most of the bladder, except the trigone.

A horseshoe kidney results from fusion of the lower poles of both kidneys during embryologic development.
The fused part forms an isthmus, usually located anterior to the aorta and inferior vena cava, and lies below the origin of the inferior mesenteric artery (IMA) — which prevents the kidney from ascending normally.
As a result, the ureters pass anterior to the isthmus, which may cause urinary stasis or obstruction.

❌ Why Others Are Wrong:

• Unilateral double kidney:
  Refers to duplication of collecting systems on one side — no fusion of poles.

• Rosette kidney:
  Not a recognized anatomic term.

• Pelvic kidney:
  A kidney that fails to ascend — not fused and no isthmus.

• Aberrant kidney:
  Refers to an ectopic or misplaced kidney, not fused lower poles.

✅ Seminal vesicle
Seminal vesicles develop from the mesonephric (Wolffian) ducts, not the urogenital sinus.

The urogenital sinus gives rise to:

• Urinary bladder

• Prostatic urethra and prostate gland

• Membranous and penile urethra in males

• Bulbourethral (Cowper’s) glands

In females, it contributes to the lower vagina and vestibular glands.

The seminal vesicle, however, is derived from an outgrowth of the mesonephric duct, which is distinct from the urogenital sinus.

Explanation of the Incorrect Options

❌ Bulbourethral gland
Derived from the urogenital sinus (outgrowth of spongy urethra).

❌ Urinary bladder
Formed from the upper part of the urogenital sinus (except the trigone, which comes from mesonephric ducts).

❌ Prostate gland
Formed from endodermal outgrowths of the prostatic urethra (urogenital sinus derivative).

❌ Penile urethra
Derived from the phallic part of the urogenital sinus.

❌ Seminal vesicle
Correct — comes from the mesonephric duct, not the urogenital sinus.

✅ Efferent ductules

Why Other Options Are Wrong

❌ Rete testis – Derived from testis cords of the gonadal ridge, not from mesonephric tubules.

❌ Seminiferous tubules – Develop from the gonadal (testicular) cords, not the mesonephric system.

❌ Straight tubules – Connect seminiferous tubules to the rete testis; also arise from gonadal cords, not mesonephric tubules.

❌ Epididymal duct – Formed from the mesonephric duct, not the mesonephric tubules.

✅ Collecting tubules

Why Other Options Are Wrong

❌ Bowman’s capsule – Develops from the metanephric blastema, not the ureteric bud.

❌ Loop of Henle – Formed from metanephric blastema; although it extends into both cortex and medulla, it is not derived from the ureteric bud.

❌ Proximal convoluted tubules – Derived from metanephric blastema and confined mainly to the cortex.

❌ Distal convoluted tubules – Also develop from metanephric blastema, located in the cortex.

✅ Pelvic part of urogenital sinus

Why Other Options Are Wrong

❌ Allantois – Gives rise to the urachus, which forms the median umbilical ligament, not the female urethra.

❌ Vesical part of urogenital sinus – Develops into most of the urinary bladder, not the urethra.

❌ Phallic part of urogenital sinus – Contributes to the vestibule of vagina in females, not the urethra.

❌ From mesonephric duct – In males, contributes to structures like epididymis, vas deferens, and seminal vesicle; not involved in female urethra formation.

https://www.pulsus.com/scholarly-articles/bilateral-malrotated-kidney-with-major-venous-variant-a-cadaver-case-report.html

During embryological development:

• Kidneys ascend from the pelvis

• At the same time, the renal hilum rotates from anterior → medial

If this rotation is abnormal or incomplete, the hilum may:

• Face anteriorly

• Posteriorly

• Or laterally

An anteriorly facing hilum is classic for:

Renal malrotation ✅

❌ Why the other options are wrong

❌ Ectopic kidney

• Refers to abnormal position, not orientation

• Hilum may still face medially

❌ No rotation of kidney

• Hilum remains anterior in no-rotation — BUT examiners use the term malrotation to describe any abnormal rotation pattern, including no rotation

❌ Renal agenesis

• Kidney absent — no hilum to see

❌ Undescended kidney

• Kidney remains low in pelvis

• Hilum orientation depends on rotation, not descent

✅ Entire urethra in females
The pelvic part of the urogenital sinus develops into the prostatic and membranous urethra in males, and the entire urethra in females.

The urogenital sinus is divided into three parts during development:

1. Vesical part → forms most of the urinary bladder (except trigone, which is mesodermal from mesonephric ducts).

2. Pelvic part → gives rise to:

   • Entire urethra in females.

   • Prostatic urethra and membranous urethra in males.

3. Phallic part → contributes to development of the penile urethra in males and the vestibule of the vagina in females.

Thus, the pelvic part specifically accounts for the female urethra in its entirety.

Explanation of the Incorrect Options

❌ Base of the bladder
Formed from the vesical part of the urogenital sinus, not the pelvic part.

❌ Clitoral primordium
Arises from the genital tubercle, not from the urogenital sinus.

❌ Genital tubercle
A separate structure that gives rise to the clitoris in females and glans penis in males.

❌ Trigone
Derived from the incorporated caudal ends of the mesonephric ducts (mesodermal origin), not from the urogenital sinus.

During embryological development, the kidneys form in the pelvic region and normally ascend to their final position in the lumbar region. In the case of a horseshoe kidney, the lower poles of the two kidneys fuse, forming a U-shaped structure. When this fused kidney tries to ascend, it gets stopped by the inferior mesenteric artery (IMA), which arises from the aorta at the level of L3. Because the fused lower poles cannot pass beyond this vessel, the ascent halts, and the kidney remains lower than normal. This is a classic embryological point and explains why horseshoe kidneys are often found in the lower lumbar region.

Explanation of the Incorrect Options

Superior mesenteric artery (SMA)
Although the SMA is also a major branch of the abdominal aorta, it originates higher (around L1). By the time the horseshoe kidney ascends, it is already restricted by the IMA before it can reach the level of the SMA. Therefore, the SMA does not play a role in limiting ascent.

Abdominal aortic bifurcation
The aortic bifurcation into the common iliac arteries occurs at the level of L4. This point is below the origin of the IMA, so it cannot act as the barrier. In fact, if ascent were blocked here, the kidney would not even reach the lumbar region, which is not what we observe.

Urinary bladder
The bladder is located in the pelvis, and while it is related to the initial position of the developing kidneys, it does not interfere with their ascent. Once the kidneys begin migrating upward, the bladder stays behind in the pelvis, playing no role in restricting movement.

Ureter
The ureters elongate as the kidneys ascend, but they are flexible tubes that adapt to the kidney’s position. They do not mechanically restrict kidney ascent.

The urachus is a fetal structure connecting the apex of the bladder to the umbilicus, which normally obliterates after birth to form the median umbilical ligament.

Failure to close completely can lead to:

1. Patent urachus (urachal fistula):

   • Entire urachus remains open

   • Urine flows from the bladder directly to the umbilicus

2. Urachal cyst:

   • Central portion remains patent, ends closed

   • Does not allow urine to escape

3. Urachal sinus:

   • Umbilical end remains patent, but bladder end closed

   • May drain mucus, but not urine

4. Renal agenesis / ectopic kidney:

   • Unrelated to urachal patency

Why the Other Options Are Incorrect:

• Urachal cyst: Closed at both ends → no urine discharge

• Urachal sinus: Closed at bladder → only mucus may drain

• Renal agenesis / ectopic kidney: Kidney anomaly, unrelated to urachal urine leakage

The ureters develop from the ureteric bud, which is an outgrowth of the mesonephric (Wolffian) duct during week 5 of embryonic development.

• The ureteric bud gives rise to:

  • Ureter

  • Renal pelvis

  • Major and minor calyces

  • Collecting ducts

Meanwhile, the metanephric mesenchyme (blastema) forms the nephrons (glomerulus, proximal tubule, loop of Henle, and distal tubule).

Why the Other Options Are Incorrect

• Ureteric bud from metanephric duct – The metanephric duct does not exist; the metanephric tissue is mesenchyme, not a duct.

• Mesenchymal condensation posterior to cloaca – This is associated with other pelvic structures, not the ureter.

• Ureteric bud from allantois – The allantois contributes to the urachus and bladder apex, not the ureter.

• None of these – Incorrect because the first option is the accurate description.

The pronephros is the most primitive of the three stages of kidney development (pronephros → mesonephros → metanephros).

• It appears early in the 4th week of gestation.

• It is nonfunctional in humans and degenerates by the end of the 4th week, while the mesonephros takes over as the interim kidney until the permanent metanephros develops around the 5th week.

Why the Other Options Are Incorrect

• End of 6th week – Too late; by then, the pronephros has already degenerated and the metanephros is beginning to mature.

• End of 3rd week – Too early; pronephros only starts to appear around the early 4th week.

• End of 5th week – Incorrect; degeneration has already occurred by then.

• End of 2nd week – Far too early; kidney structures are not forming yet.

• Renal vein –

• • The renal veins drain blood from the kidney after it has already developed in its proper position.

  • They are not located inferior enough to block the ascent.

Thus, they have no role in stopping a horseshoe kidney.

• Renal artery –

• The renal arteries actually form after the kidney reaches its final position in the upper abdomen.

• They branch directly from the abdominal aorta at ~L1–L2.

Since they don’t exist during the ascent, they cannot be the obstacle.

• Inferior mesenteric artery (IMA) –

• The IMA originates at the level of L3, which is below the normal final kidney position (L1–L2).

• When the lower poles of the fused kidney try to ascend, they hit the IMA and can’t move further up.

That’s why horseshoe kidneys are usually found lower in the abdomen (around L3–L5).

• Superior mesenteric artery (SMA) –

• The SMA arises higher, at L1.

• By the time kidneys reach this level, they are already fused and stuck below the IMA.

So the SMA is too high to be the obstructing vessel.

• Celiac trunk –

• The celiac trunk is the highest of all these vessels, arising around T12.

• Kidneys never ascend this high (they normally stop at L1–L2).

• Therefore, it plays no role in stopping ascent.

Answer Breakdown:

• Renal vein →  drains kidney, no obstruction

• Renal artery → ❌ forms later, not during ascent

• Inferior mesenteric artery → ✅ blocks fused kidney’s ascent

• Superior mesenteric artery → ❌ too high, not the obstacle

• Celiac trunk → ❌ way too high, unrelated

• The urogenital sinus has three regions:

  • Vesical part → Forms most of the urinary bladder.

  • Pelvic part → Forms the prostatic urethra in males and the entire urethra in females.

  • Phallic part → Contributes to penile urethra in males and vestibule of vagina in females.

Other options:

• Vagina – Incorrect: Mainly derived from the vaginal plate (endoderm of urogenital sinus + paramesonephric ducts).

• Fallopian tubes – Incorrect: Derived from paramesonephric (Müllerian) ducts, not urogenital sinus.

• Ureter – Incorrect: Develops from the ureteric bud (outgrowth of mesonephric duct).

• Kidneys – Incorrect: Develop from metanephric blastema + ureteric bud, not urogenital sinus.

• The urinary bladder develops mainly from the vesical part of the urogenital sinus (which gives rise to most of the bladder lining). This portion is highly distensible.

• However, the trigone (the triangular area between the two ureteric orifices and the internal urethral orifice) has a different embryological origin.

• It is formed by the incorporation of the lower ends of the mesonephric ducts into the posterior wall of the developing bladder.

• Since the trigone is derived from mesoderm (mesonephric duct), unlike the rest of the bladder (endodermal origin from urogenital sinus), it is the least distensible part.

• With time, even the trigone becomes overgrown by endodermal epithelium, but its mesodermal origin makes it distinct functionally.

Answer breakdown:

• Phallic part of urogenital sinus → gives rise to penile urethra, not bladder.

• Pelvic part of urogenital sinus → contributes to prostatic and membranous urethra.

• Allantois → becomes urachus → median umbilical ligament.

• Mesonephric duct → correct (forms trigone, least distensible part).

• Vesical part of urogenital sinus → forms most of bladder, highly distensible.

The ureteric bud is an outgrowth from the mesonephric (Wolffian) duct during kidney development. It interacts with the metanephric mesenchyme (blastema) to form the definitive kidney.

Structures derived from the ureteric bud:

• Ureter → The stalk of the bud forms the ureter.

• Renal pelvis → The proximal part of the bud dilates to form the pelvis.

• Calyces → Formed from the branching of the ureteric bud.

• Collecting ducts → Terminal branches of the ureteric bud become collecting ducts.

Structures not derived from the ureteric bud:

• Bladder → Derived from the urogenital sinus, not the ureteric bud. Only the trigone of the bladder has contributions from the mesonephric ducts, but the main bladder wall comes from the urogenital sinus endoderm.

✅ Correct Answer: Bladder

❌ Why the other options are incorrect

• Ureter → Directly develops from the stalk of ureteric bud.

• Calyx → Formed by branching of ureteric bud.

• Renal pelvis → Proximal ureteric bud dilates to form the pelvis.

• Collecting duct → Terminal branches of ureteric bud give rise to collecting ducts.

✅ Correct Answer: Ectoderm

Explanation:

Step 1: Urethral development in males

• The male urethra develops from two sources:

  • Pelvic part (proximal urethra): from the endodermal urogenital sinus.

  • Phallic/distal part (in the glans penis): from the ectodermal cells of the urethral plate, which canalize to form the navicular fossa.

Step 2: Clinical relevance

• Errors in this process can lead to hypospadias, where the urethra opens on the ventral side of the penis.

❌ Why the other options are incorrect:

Endoderm

• ❌ Forms the proximal pelvic urethra, not the distal urethra in the glans.

Lateral plate mesoderm

• ❌ Contributes to vascular and connective tissue, not urethral lining.

Intermediate mesoderm

• ❌ Gives rise to kidneys and gonads, not urethral epithelium.

Paraxial mesoderm

• ❌ Forms musculoskeletal structures, not urethra.

The penile (spongy) urethra is derived mainly from the phallic part of the urogenital sinus.

• During development, the phallic part elongates and contributes to the urethral plate.

• This plate canalizes and forms the spongy urethra that runs through the corpus spongiosum of the penis.

• The only exception is the terminal portion of the glanular urethra, which is formed by an ectodermal ingrowth from the tip of the glans.

Thus, the bulk of the penile urethra comes from the phallic part of the urogenital sinus.

Why the Other Options Are Incorrect

• Surface ectoderm ❌
  This forms only the distal glanular urethra (via ectodermal ingrowth), not the main penile urethra.

• None of these ❌
  Incorrect, because the phallic part is indeed responsible.

• Vesical part of urogenital sinus ❌
  This gives rise to the urinary bladder, not the penile urethra.

• Pelvic part of urogenital sinus ❌
  This contributes to the prostatic and membranous urethra, not the penile urethra.

The male urethra has different embryological origins depending on the segment:

• The prostatic urethra (the widest part) and the membranous urethra (the narrowest/least dilated part) develop from the pelvic part of the urogenital sinus.

• The spongy/penile urethra develops from the phallic part of the urogenital sinus, except for its very terminal portion.

• The glanular urethra (terminal tip at external meatus) develops from an ingrowth of surface ectoderm.

Since the least dilated part of the male urethra is the membranous urethra, its origin is the pelvic part of the urogenital sinus.

Why the Other Options Are Incorrect

• Allantois ❌
  This gives rise to the urachus, which later becomes the median umbilical ligament, not the urethra.

• Surface ectoderm ❌
  Only contributes to the terminal glanular urethra, not the membranous portion.

• Vesical part of urogenital sinus ❌
  This forms the urinary bladder, not the urethra.

• Phallic part of urogenital sinus ❌
  This forms the spongy urethra, but not the membranous (least dilated) part.

• The urachus is a fetal remnant of the allantois, connecting the apex of the urinary bladder to the umbilicus.

• Normally, after birth, it obliterates to form the median umbilical ligament, a fibrous cord running in the midline from the bladder to the umbilicus.

Umbilicus ❌

• This is the navel itself, not a remnant of the urachus. It is where the umbilical cord was attached, not derived from urachal tissue.

Medial umbilical ligament ❌

• These are the remnants of the obliterated umbilical arteries, not the urachus. They run on either side of the bladder.

Lateral umbilical ligament ❌

• These are folds that cover the inferior epigastric vessels. They are not embryological remnants, but peritoneal folds.

Umbilical artery ❌

• In the fetus, the umbilical arteries carry deoxygenated blood to the placenta. After birth, they become the medial umbilical ligaments, not related to the urachus.

• The urachus is a fetal structure connecting the apex of the bladder to the umbilicus (a remnant of the allantois). Normally, it obliterates and forms the median umbilical ligament.

• If the urachus remains completely patent, it results in a urachal fistula: an abnormal connection between the bladder and umbilicus.

• Clinical feature: urine discharge from the umbilicus in infants or children.

Urachal sinus ❌

• Occurs when the urachus obliterates at the bladder end but remains open at the umbilical end.

• Results in a blind pouch opening at the umbilicus, but not communicating with the bladder.

Urachal cyst ❌

• Occurs when a small part of the urachus remains patent in the middle, but both ends are closed.

• Forms a fluid-filled cyst in the midline between bladder and umbilicus.

Urogenital sinus ❌

• A normal embryological cavity that gives rise to parts of the urinary bladder, urethra, and genitalia.

• It is not a pathological remnant of the urachus.

None of these ❌

• Incorrect because the correct anomaly described is clearly a urachal fistula.

Clinical Scenario Recap

A 4-year-old girl has continuous dribbling of urine, and on examination, urine is coming out of the vagina.
This suggests that urine is not following the normal route (via bladder → urethra) but instead is being misdirected due to an embryological anomaly involving abnormal connection of the ureter.

Correct Answer: Ureter opening into the urogenital sinus ✅

• In normal development, the ureteric bud (which becomes the ureter) should separate properly from the mesonephric duct and establish its connection with the urinary bladder.

• If this process is faulty, the ureter may end up opening abnormally — sometimes into the vagina, vestibule, or urethra.

• In females, such an ectopic ureter often drains below the sphincteric mechanism → leading to continuous incontinence (because urine bypasses bladder storage and sphincter control).

• Thus, the anomaly described (urine exiting through the vagina) is best explained by ureter opening into the urogenital sinus.

Why not the other options?

1. Formation of vaginal fistula ❌

   • A fistula is an acquired abnormal communication (e.g., vesicovaginal fistula after obstructed labor or surgery).

   • In this case, the problem is congenital (present from birth), not acquired.

2. Ureter opening into the base of urinary bladder ❌

   • This is the normal anatomical location.

   • If the ureter opened normally, urine would drain into the bladder → no continuous leakage.

3. Ureter opening into cloaca ❌

   • The cloaca exists only in early embryonic life (common opening for urinary, genital, and GI tracts).

   • Normally, it divides into the urogenital sinus and anorectal canal. Persistence of cloaca would cause more severe anomalies, not isolated vaginal leakage.

4. Ureter opening into paramesonephric duct ❌

   • The paramesonephric (Müllerian) duct develops into uterus, fallopian tubes, and part of vagina.

   • The ureter has no embryological association with this duct.

     Key Takeaway

     The scenario highlights ectopic ureter in a female child → a classic cause of continuous urinary dribbling with normal voiding pattern.

• The ureteric bud is an outgrowth from the mesonephric (Wolffian) duct that gives rise to the collecting system of the kidney: ureter, renal pelvis, calyces, and collecting ducts.

• If the ureteric bud duplicates or branches abnormally, it can result in a double ureter anomaly.

• This can lead to partial or complete duplication of the ureter, sometimes associated with abnormal drainage into the bladder or ectopic sites.

Why the Other Options Are Wrong

Metanephros ❌

• Forms the nephrons (glomeruli, proximal tubules, loop of Henle, distal tubules).

• Does not directly give rise to the ureter.

Pronephros ❌

• The earliest, rudimentary kidney stage in development.

• It regresses and does not contribute to permanent structures.

Mesonephros (Wolffian duct) ❌

• Gives rise to parts of the male genital tract (epididymis, vas deferens, seminal vesicles).

• The ureteric bud sprouts from this duct but the duct itself does not duplicate to cause double ureter.

Paramesonephric duct ❌

• Also called Müllerian duct.

• Forms the female reproductive tract (fallopian tubes, uterus, upper vagina).

• Not related to urinary tract development.

Bilateral renal agenesis → complete absence of both kidneys, causing severe oligohydramnios, which leads to Potter sequence:

• Respiratory distress (lung hypoplasia)

• Flattened facial features (“Potter facies”)

• Limb deformities

This is a classic presentation.

✔ Why This Option Is Correct

Bilateral renal agenesis — ✅

• No kidneys → almost no fetal urine → severe oligohydramnios

• Oligohydramnios causes lung hypoplasia, leading to respiratory distress.

• Characteristic flattened face, low-set ears, limb deformities = Potter sequence.

• Always associated with very poor prognosis.

❌ Why The Other Options Are Incorrect

Renal hypoplasia — ❌

• Kidneys are small but present.

• Does not cause complete absence of urine → oligohydramnios would not be severe enough to produce classic Potter features.

Polycystic kidney disease — ❌

• Kidneys are enlarged with cysts, not absent.

• Oligohydramnios may occur in autosomal recessive PKD, but kidneys are visible on ultrasound.

Ectopic kidney — ❌

• Kidney(s) are present but misplaced (pelvic kidney).

• Amniotic fluid remains normal.

• No facial or pulmonary abnormalities.

Hydronephrosis — ❌

• Kidney is present but dilated.

• Does NOT cause severe oligohydramnios unless bilateral and extreme — still kidneys visible.

Image Reference :- https://www.mayoclinic.org/diseases-conditions/bladder-exstrophy/symptoms-causes/syc-20391299

When the urethra opens on the dorsal surface AND the bladder is exposed/protruding onto the abdominal wall, this combination strongly indicates bladder exstrophy — a severe defect of the ventral body wall + pelvic region caused by abnormal closure of the anterior abdominal wall and failure of mesoderm migration.

Epispadias is part of bladder exstrophy, but when the bladder is open externally, the diagnosis becomes exstrophy of the bladder.

Exstrophy of bladder — ✅

• Caused by failure of the lateral body wall folds to close properly.

• Characterized by:

  • Dorsal opening of urethra (epispadias)

  • Exposure of bladder mucosa on the abdominal wall

• This is the classical presentation described in the question.

❌ Why the Other Options Are Incorrect

Hypospadias — ❌

• Urethral opening is on the ventral (underside) of penis.

• No bladder involvement.

• Opposite orientation from what is described.

Epispadias — ❌

• Urethral opening is on the dorsal surface, correct

• BUT there is no protruding bladder in isolated epispadias.

• When bladder protrudes → it is not just epispadias anymore → it is bladder exstrophy.

Exstrophy of cloaca — ❌

• More severe than bladder exstrophy.

• Includes imperforate anus, omphalocele-like defects, and exposure of both urinary & intestinal mucosa.

• Not described here.

Undescended testes — ❌

• Completely unrelated to bladder or urethral opening defects.

• No genital surface malformation of this type.

A horseshoe kidney forms when the lower poles of the kidneys fuse.
During ascent, the fused kidney gets trapped under the inferior mesenteric artery, preventing it from reaching its normal lumbar position.

✔ Why This Option Is Correct

Inferior mesenteric artery — ✅

• The IMA arises at L3, exactly where the fused lower poles of a horseshoe kidney get blocked.

• This is the classic and most tested cause of failed kidney ascent.

• As a result, the kidney remains low in the abdomen, usually around L3–L5.

❌ Why the Other Options Are Incorrect

Inferior vena cava — ❌

• Lies to the right of the aorta and does not obstruct midline ascent.

• No classical association with horseshoe kidney.

Superior mesenteric artery — ❌

• Originates much higher (L1).

• By the time the kidney reaches this level, the ascent would already have been blocked earlier if fused.

Aortic bifurcation — ❌

• Occurs at L4, below the IMA.

• The kidney would already have been blocked earlier at the IMA — this is too low to be responsible.

Median sacral artery — ❌

• Arises at the posterior aspect of the aortic bifurcation.

• Too small and too low to impede ascent.

The urinary bladder develops mainly from the vesical part of the urogenital sinus, which arises after the cloaca is divided by the urorectal septum.

This part forms almost the entire bladder, except the trigone region (which is mesonephric duct–derived initially but gets overgrown).

Urogenital sinus (vesical part) — ✅

• Forms most of the urinary bladder (epithelium + mucosa).

• Specifically, the superior and main body of the bladder.

• Smooth muscle of the bladder wall forms from splanchnic mesoderm, but the epithelium is from the urogenital sinus.

❌ Why The Other Options Are Incorrect

Mesonephric duct — ❌

• Only contributes temporarily to the trigone region.

• Later, trigone becomes overgrown by endoderm of the urogenital sinus.

• It does NOT form the whole bladder.

Metanephric blastema — ❌

• This forms nephrons of the definitive kidney, not the bladder.

• No part of the bladder is derived from this tissue.

Cloacal membrane — ❌

• This is merely the membrane covering the cloaca (ectoderm + endoderm).

• It ruptures to create openings but does not form the bladder itself.

Allantois — ❌

• The allantois becomes the urachus, then the median umbilical ligament.

• Only forms the apex of the bladder’s fibrous connection, not the bladder body.