## **Core Concept**
The question tests the understanding of the neural pathway involved in the pupillary light reflex. The pupillary light reflex involves afferent input from the retina to the midbrain and efferent output from the midbrain to the pupils. The afferent pathway involves the optic nerve (cranial nerve II), while the efferent pathway involves the oculomotor nerve (cranial nerve III).

## **Why the Correct Answer is Right**
The correct answer, cranial nerve III (oculomotor nerve), is implicated because the lesion affects the efferent pathway of the pupillary light reflex. The absence of both direct and consensual light reflexes suggests a problem with the motor response rather than the sensory input. The oculomotor nerve controls the constriction of the pupil; a lesion in this nerve would prevent the pupil from constricting in response to light, affecting both the direct (ipsilateral) and consensual (contralateral) reflexes.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The optic nerve (cranial nerve II) is involved in the afferent pathway of the pupillary light reflex. A lesion in the optic nerve would affect the afferent input, leading to an impaired consensual reflex on the contralateral side but not affecting the direct reflex on the ipsilateral side.
- **Option B:** This option does not correspond to a cranial nerve directly related to the described reflexes.
- **Option D:** The trochlear nerve (cranial nerve IV) and the abducens nerve (cranial nerve VI) are involved in eye movements, not in the pupillary light reflex.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that a lesion in the afferent pathway (optic nerve) affects the consensual but not the direct light reflex on the ipsilateral side, whereas a lesion in the efferent pathway (oculomotor nerve) affects both direct and consensual light reflexes on the ipsilateral side.

## **Correct Answer:** . oculomotor nerve