Question

All of the following are seen in third (3rd) cranial nerve palsy EXCEPT?

• A. Mydriasis
• B. Loss of light reflex
• C. Loss of abduction
• D. Ptosis

Hint:

Which muscle performs abduction, and which cranial nerve supplies it?

Answer 1

The Correct Option is C

Step 1: The oculomotor (third) nerve supplies the levator palpebrae superioris, the superior rectus, medial rectus, inferior rectus, and inferior oblique muscles, plus the parasympathetic fibres to the sphincter pupillae and ciliary muscle. A complete palsy therefore knocks out all these functions.

Step 2: This explains the features that ARE seen: ptosis from levator weakness, a fixed dilated pupil (mydriasis) and loss of the light reflex from parasympathetic loss, and a down and out eye position.

Step 3: The question asks for the exception. Abduction of the eye is carried out by the lateral rectus, which is supplied by the sixth (abducens) nerve, not the third nerve. So loss of abduction is NOT a feature of third nerve palsy and is the correct answer to choose.

Step 4: In fact, in third nerve palsy abduction is preserved or even appears prominent because the unopposed lateral rectus pulls the eye outward, giving the characteristic down and out eye.