This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Orbit, Subject name: Anatomy
Ptosis is due to: 
Select and explain. 
A. 3rd CN palsy, B. 4th CN palsy, C. 5th CN palsy, D. 6th CN palsy
A: 3rd CN palsy
Answer: A: 3rd CN palsy 
Explanation: Ans. (a) 3rd CN palsy (oculomotor)Ref.:BDC 6th ed. Vol-III/361-62* Ptosis is drooping of eyelid.* Eyelid is comprised of 4 muscles:Muscles, their innervation and functionMuscleCN innervationFunction* LPSIIIOpens the eyelid* Muller's muscleT1 (NOT a CN; sympathetic nerve)Opens eyelid when LPS tired* FrontalisVIICloses the eyelid* Orbicularis oculiVIICloses the eyelid* CN III palsy leads to drooping of eyelid (Ptosis).* CN VII palsy may also cause ptosis.Also Know*All the extraocular muscles are supplied by CN III except lateral rectus and superior oblique.* Lateral rectus supplied by- CN VI* Superior oblique supplied by- CN IVRemember- LR6; So4