✅ Correct Answer: Palpebral part of orbicularis oculi
Actually, not quite! → The truly correct answer should be levator palpebrae superioris, but since it’s not listed, the closest relevant structure is the palpebral part of orbicularis oculi—and that’s a trick here, because neither orbicularis oculi nor its parts are elevators of the eyelid; rather, they close the eyelid.
Therefore, to understand what’s wrong with the options, we need to walk through them carefully.
🎓 Explanation:
Ptosis = drooping of the upper eyelid.
→ To identify the muscle affected, we need to know which muscle elevates the upper eyelid.
1️⃣ Main elevator of the upper eyelid:
2️⃣ Additional minor elevator:
Now let’s evaluate each option:
Option |
Role |
Relation to eyelid drooping |
A) Orbital part of orbicularis oculi |
Closes the eyelid forcefully (squinting); not responsible for elevation |
Paralysis would impair closure, not cause ptosis |
B) Palpebral part of orbicularis oculi |
Closes eyelid gently (blinking); also not an elevator |
Damage → incomplete closure, not ptosis |
C) Inferior tarsus |
This is a connective tissue plate, not a muscle |
Not responsible for movement → irrelevant |
D) Medial rectus |
Moves eyeball medially → no function in eyelid movement |
Damage → eye movement deficit, no ptosis |
E) Superior rectus |
Moves eyeball upward → no direct role in eyelid elevation |
Damage → impaired upward gaze, no ptosis |
👉 What’s missing?
The actual muscle causing ptosis is levator palpebrae superioris, innervated by CN III.
Alternatively, sympathetic damage → ptosis via superior tarsal muscle (as in Horner’s syndrome).
Since none of these muscles are elevators, none of the options directly cause ptosis if paralyzed.
❌ Why each option is wrong (and what they actually do):
-
Orbital part of orbicularis oculi:
→ closes eye tightly (squinting); damage → eye stays open, not ptosis.
-
Palpebral part of orbicularis oculi:
→ closes eye gently; damage → impaired blinking, not ptosis.
-
Inferior tarsus:
→ structural tissue, no muscle action.
-
Medial rectus:
→ moves eye medially; no action on eyelid.
-
Superior rectus:
→ moves eye upward; no eyelid action.
✅ Therefore, none of these are correct if we strictly stick to anatomy.
However, in an exam setting, if forced to choose, the palpebral part of orbicularis oculi is the least incorrect because it at least relates to eyelid movement, even if it causes closure instead of elevation.
📝 Key takeaway:
-
Ptosis = dysfunction of levator palpebrae superioris (CN III) or superior tarsal muscle (sympathetics).
-
Orbicularis oculi closes the eyelid; paralysis = inability to close, not drooping.