## **Core Concept**
The question tests understanding of **ocular motor nerves** and their functions, particularly in controlling eye movements. The inability to adduct one eye while looking to the right suggests a problem with the nerve responsible for medial rectus muscle function, which is crucial for adduction.

## **Why the Correct Answer is Right**
The correct answer, **CN III (Oculomotor nerve)**, is right because the oculomotor nerve innervates the medial rectus muscle, which is responsible for adduction of the eye. A lesion in CN III would lead to weakness or paralysis of the medial rectus muscle, making it difficult or impossible to adduct the affected eye. This is consistent with the patient's inability to adduct his left eye while looking to the right.

## **Why Each Wrong Option is Incorrect**
- **Option A:** A lesion in the **Abducens nerve (CN VI)** would affect the lateral rectus muscle, leading to an inability to abduct the eye, not adduct it.
- **Option B:** A problem in the **Trochlear nerve (CN IV)** would affect the superior oblique muscle, which primarily helps in intorsion and secondarily in depression and abduction of the eye, not adduction.
- **Option D:** The **Medial longitudinal fasciculus (MLF)** is involved in internuclear ophthalmoplegia when damaged, which could also present with an inability to adduct an eye. However, the question specifically points towards a nerve lesion rather than a central pathway issue.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that for conjugate gaze (both eyes moving in the same direction), **CN III, IV, and VI** work together. A lesion in CN III affects adduction, while a lesion in CN VI affects abduction. The MLF connects these nuclei and is crucial for conjugate eye movements. A clinical scenario like this can also hint at **internuclear ophthalmoplegia** if the issue was with MLF, but the provided options and correct answer focus on a **peripheral nerve** lesion.

## **Correct Answer:** . CN III (Oculomotor nerve)