**Question:** A 30 year old man came to the outpatient department because he had suddenly developed double vision. On examination it was found that his right eye, when at rest, was turned medially. The most likely anatomical structures involved are -

A. Oculomotor nerve III
B. Trochlear nerve IV
C. Abducens nerve VI
D. Optic nerve II

**Core Concept:**
The question is asking about the most likely anatomical structures involved in causing a medial gaze palsy, which is characterized by a medial or inward deviation of an eye when in a neutral or resting position. This condition is commonly seen in medical students and residents and helps to test their knowledge of the cranial nerves and their functions.

**Why the Correct Answer is Right:**
The correct answer is **B. Trochlear nerve IV** because the trochlear nerve controls the superior rectus muscle, which is responsible for medial rectus function. This muscle is responsible for keeping the eye in the correct position when the eye is at rest. A lesion affecting the trochlear nerve would lead to a medial gaze palsy, causing the eye to deviate inwards when at rest.

**Why Each Wrong Option is Incorrect:**
A. **Oculomotor nerve III (Cranial nerve III)** controls the inferior rectus and superior oblique muscles. These muscles are responsible for downward and upward eye movements, respectively. An oculomotor nerve palsy would not cause a medial deviation of the eye at rest.

C. **Abducens nerve VI (Cranial nerve VI)** controls the lateral rectus muscle, which is responsible for lateral eye movements. A lesion affecting the abducens nerve would not cause a medial deviation at rest.

D. **Optic nerve II (Cranial nerve II)** controls the pupillary light reflex and the iris sphincter and dilator muscles. An optic nerve lesion would not cause a medial deviation of the eye at rest.

**Clinical Pearl:**
Recall that the medial rectus muscle is the only muscle that controls the eye's position at rest. Understanding the cranial nerves and their functions is crucial for diagnosing and treating neurological disorders. Medical students and residents often encounter patients with cranial nerve palsies, and this question is designed to test your knowledge of these structures and their functions.