Question:medium
All of the following are seen in third (3rd) cranial nerve palsy EXCEPT?
All of the following are seen in third (3rd) cranial nerve palsy EXCEPT?
Show Hint
Which muscle performs abduction, and which cranial nerve supplies it?
Updated On: Jun 24, 2026
- Mydriasis
- Loss of light reflex
- Loss of abduction
- Ptosis
Show Solution
The Correct Option is C
Solution and Explanation
Step 1: Map the muscles of the third nerve first: it drives the eyelid lifter (levator), four of the extraocular muscles (superior, medial, inferior recti and inferior oblique), and through its parasympathetic supply the pupil constrictor and accommodation. The odd one out must be a function it does NOT control.
Step 2: Walk through the options. Ptosis follows levator paralysis, mydriasis and an absent light reflex follow loss of the pupil parasympathetics, so three of the four choices are genuine signs of the palsy.
Step 3: Abduction, however, is the job of the lateral rectus, and that muscle is wired to the sixth cranial nerve. Since the third nerve never moves the eye outward, abduction stays intact, so loss of abduction cannot be a sign of third nerve palsy.
Step 4: This is why the affected eye sits down and out: with the medial rectus weak, the healthy lateral rectus drags it outward, and abduction is if anything preserved. Therefore the exception is loss of abduction.
\[\boxed{\text{Loss of abduction}}\]
Step 2: Walk through the options. Ptosis follows levator paralysis, mydriasis and an absent light reflex follow loss of the pupil parasympathetics, so three of the four choices are genuine signs of the palsy.
Step 3: Abduction, however, is the job of the lateral rectus, and that muscle is wired to the sixth cranial nerve. Since the third nerve never moves the eye outward, abduction stays intact, so loss of abduction cannot be a sign of third nerve palsy.
Step 4: This is why the affected eye sits down and out: with the medial rectus weak, the healthy lateral rectus drags it outward, and abduction is if anything preserved. Therefore the exception is loss of abduction.
\[\boxed{\text{Loss of abduction}}\]
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