**Question:** Nerve compressed by aneurysm of posterior communicating artery is -

A. Optic Nerve
B. Oculomotor Nerve
C. Trigeminal Nerve
D. Pons

**Correct Answer:** **B. Oculomotor Nerve**

**Core Concept:** Posterior communicating artery (PCoA) is one of the branches of the internal carotid artery that supplies blood to the brain, specifically the region surrounding the optic chiasm and the lateral part of the cavernous sinus.

**Why the Correct Answer is Right:** An optic nerve (A) and trigeminal nerve (C) are not typically compressed by aneurysms of the posterior communicating artery, as they are supplied by different arteries (optic nerve by the posterior cerebral artery and trigeminal nerve by the internal carotid artery). Instead, the oculomotor nerve (B) is more susceptible to compression due to its course. The oculomotor nerve passes through the cavernous sinus and passes under the posterior communicating artery, making it vulnerable to compression when the PCoA enlarges due to an aneurysm.

**Why Each Wrong Option is Incorrect:**

A. Optic Nerve: This nerve is supplied by the posterior cerebral artery and is not compressed by aneurysms of the posterior communicating artery.
B. Oculomotor Nerve: As mentioned above, the oculomotor nerve is more susceptible to compression due to its course but not because of the posterior communicating artery.
C. Trigeminal Nerve: This nerve is supplied by the internal carotid artery and is not compressed by aneurysms of the posterior communicating artery.
D. Pons: The pons is a region of the brainstem and is not directly compressed by aneurysms of the posterior communicating artery.

**Clinical Pearl:** In cases of aneurysm of the posterior communicating artery, it is essential to consider the potential compression of nearby nerves, such as the oculomotor nerve, to avoid misdiagnosis and unnecessary complications.