**Question:** Third nerve palsy with pupillary sparing is seen in all except:

A. Atherosclerosis
B. Hypertension
C. Brainstem stroke
D. Orbital cellulitis

**Core Concept:**
Third nerve palsy is a neurological condition characterized by the dysfunction of the third cranial nerve (also known as the oculomotor nerve). This nerve controls the movements of the eye muscles, including the opening and closing of the eyelids, and the dilation and constriction of the pupils. Pupillary sparing refers to the preserved function of the pupillary constriction reflex, which is controlled by the parasympathetic system innervating the pupil.

**Why the Correct Answer is Right:**
In the context of third nerve palsy, the correct answer is D (orbital cellulitis). Orbital cellulitis refers to an infection in the tissues surrounding the eye, typically caused by bacteria. In such cases, the oculomotor nerve (third nerve) is commonly affected due to its proximity to the affected area.

**Why Each Wrong Option is Incorrect:**

A. Atherosclerosis (hardening of the arteries): This is a condition affecting the large arteries, including the carotid arteries supplying blood to the brain. Atherosclerosis does not specifically affect the oculomotor nerve, making it an incorrect answer.

B. Hypertension (high blood pressure): Although hypertension can lead to various neurological manifestations, it does not specifically affect the oculomotor nerve. Therefore, option B is incorrect.

C. Brainstem stroke: Brainstem strokes can lead to a range of neurological deficits, including third nerve palsy. However, the specific cause of the palsy is not mentioned in this case. Hence, option C is not entirely correct, as the answer should differentiate the cause of the third nerve palsy.

**Clinical Pearl:**
Pupillary sparing is a valuable clue to differentiate certain causes of third nerve palsy. It allows us to narrow down the possible etiologies, as discussed in the options above. Pupillary sparing in a third nerve palsy can help differentiate between a peripheral cause (e.g., demyelination) and a central cause (e.g., brainstem stroke). This distinction is crucial for selecting the appropriate treatment and managing the patient's prognosis.