**Core Concept**
The patient's symptoms, specifically the inability to adduct or abduct the right eye and the left eye, point to a problem with the lateral rectus muscle or its innervation. The lateral rectus muscle is responsible for abducting the eye, and its function is controlled by the abducens nerve (CN VI). This nerve is unique in that it has a bilateral brainstem representation, with the majority of the fibers arising from the left abducens nucleus and crossing over to the right, while the remaining fibers arise from the right abducens nucleus and remain on the right.

**Why the Correct Answer is Right**
The correct answer, **Option A: Left abducens nerve**, is the most likely location of the patient's lesion because the abducens nerve is responsible for controlling the lateral rectus muscle, which is impaired in this patient. The left abducens nerve is responsible for innervating the right lateral rectus muscle, as the majority of the fibers from the left abducens nucleus cross over to the right. A lesion in the left abducens nerve would therefore result in impaired abduction of the right eye. The patient's inability to adduct the left eye suggests a problem with the medial longitudinal fasciculus (MLF), but this is a secondary effect of the primary lesion in the left abducens nerve.

**Why Each Wrong Option is Incorrect**
**Option B: Left abducens nucleus and left medial longitudinal fasciculus** is incorrect because a lesion in the left abducens nucleus would result in impaired innervation of both the right and left lateral rectus muscles, but the patient's symptoms only suggest impairment of the right eye. Additionally, a lesion in the MLF would result in internuclear ophthalmoplegia, which is characterized by impaired adduction of the affected eye and nystagmus of the abducting eye, not impaired abduction of the right eye.

**Option C: Right abducens nerve** is incorrect because a lesion in the right abducens nerve would result in impaired innervation of the right lateral rectus muscle, but the patient's symptoms also suggest impaired adduction of the left eye, which is not consistent with a lesion in the right abducens nerve.

**Option D: Right abducens nucleus and right medial longitudinal fasciculus** is incorrect because a lesion in the right abducens nucleus would result in impaired innervation of only the right lateral rectus muscle, and a lesion in the right MLF would result in internuclear ophthalmoplegia, but the patient's symptoms do not suggest impaired innervation of the right lateral rectus muscle.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the abducens nerve has a bilateral brainstem representation, with the majority of the fibers arising from the left abducens nucleus and crossing over to the right. This means that a lesion in the left abducens nerve can result in impaired innervation of the right lateral rectus muscle, and a lesion in the right abducens nerve can result in impaired innervation of only the right lateral rectus muscle.

**✓ Correct Answer: A. Left abducens nerve. The left abducens nerve is the most likely location of the patient’s lesion.**