A previously healthy 50 year old woman comes to the physician because of double vision for three days. Her temperature is 37 C (98.6 F). The patient denies nausea or vomiting. Examination reveals ptosis and slight divergence of the right eye. Extraocular movements are limited in all directions, except laterally. The right pupil is larger than the left and poorly reactive to light. Examination of the fundus fails to reveal papilledema. Which of the following is the most likely underlying condition?

**Core Concept**
The patient's symptoms of double vision, ptosis, and limited extraocular movements, along with a poorly reactive pupil, suggest a third cranial nerve (oculomotor nerve) palsy. This condition is characterized by the impaired function of the oculomotor nerve, which controls several extraocular muscles and the constriction of the pupil.

**Why the Correct Answer is Right**
The clinical presentation of a third cranial nerve palsy is often due to a posterior communicating artery aneurysm compressing the oculomotor nerve. This compression can lead to the impairment of the nerve's function, resulting in the observed symptoms. The aneurysm typically causes a dilated pupil that is poorly reactive to light, along with ptosis and limited extraocular movements, particularly in the upward and downward gaze. The presence of a normal temperature and the absence of nausea or vomiting help to rule out other potential causes such as a pituitary apoplexy.

**Why Each Wrong Option is Incorrect**
**Option A:** A fourth cranial nerve (trochlear nerve) palsy typically presents with an isolated superior oblique muscle weakness, causing diplopia on downward gaze, which does not match this patient's presentation.

**Option B:** A sixth cranial nerve (abducens nerve) palsy would result in weakness of the lateral rectus muscle, causing esotropia and diplopia on lateral gaze, but would not affect the pupil's reactivity.

**Option C:** A myasthenia gravis presentation is more likely to involve fluctuating weakness of multiple extraocular muscles and can be associated with ptosis, but would not typically cause a poorly reactive pupil.

**Clinical Pearl / High-Yield Fact**
A key feature of third cranial nerve palsy is the presence of a dilated pupil that is poorly reactive to light, which is often due to the compression of the oculomotor nerve by a posterior communicating artery aneurysm.

**Correct Answer:** C.

✓ Correct Answer: A. Aneurysm of the posterior communicating aery