A child age 5 years presents with loss of vision and mild proptosis of left eye. On examination direct reflex is absent but consensual reflex is present in left eye. He is having:
**Core Concept**
The patient's presentation is suggestive of a third cranial nerve palsy, which involves the oculomotor nerve. This nerve controls several extraocular muscles, including the medial rectus, superior rectus, inferior rectus, and inferior oblique, as well as the levator palpebrae superioris muscle, which elevates the eyelid. A third nerve palsy can lead to diplopia, ptosis, and loss of pupillary light reflex.
**Why the Correct Answer is Right**
The absence of the direct pupillary reflex and the presence of the consensual reflex in the affected eye indicate an afferent pupillary defect. This is due to the involvement of the afferent fibers of the pupillary light reflex pathway, which are carried by the oculomotor nerve. The affected nerve fibers are unable to transmit the light stimulus to the brain, resulting in an abnormal pupillary response.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the patient's presentation of third nerve palsy and pupillary defect.
* **Option B:** A fourth cranial nerve palsy would affect the trochlear nerve, which controls the superior oblique muscle, and would not cause an afferent pupillary defect.
* **Option C:** A sixth cranial nerve palsy would affect the abducens nerve, which controls the lateral rectus muscle, and would not cause an afferent pupillary defect.
**Clinical Pearl / High-Yield Fact**
In a patient with a third nerve palsy, the presence of an afferent pupillary defect suggests that the lesion is likely to be located in the midbrain, where the oculomotor nerve fibers are close to the red nucleus and the substantia nigra.
**Correct Answer: C. Third nerve palsy with afferent pupillary defect.**