A 32-year-old woman is admitted to the hospital after losing consciousness and collapsing in the middle of the street. A neurologic examination reveals absence of the accommodation reflex of her right eye. Which of the following is most likely involved in the pathology in this patient?
**Core Concept**
The accommodation reflex is a complex response that involves the coordinated action of the ciliary muscles, the lens, and the oculomotor nerve. It allows the eye to focus on objects at varying distances by changing the shape of the lens.
**Why the Correct Answer is Right**
The absence of the accommodation reflex in the patient's right eye suggests an abnormality in the oculomotor nerve (cranial nerve III), which is responsible for transmitting parasympathetic signals to the ciliary muscles. This nerve also controls the constriction of the pupil. The oculomotor nerve has three branches: somatic efferent, parasympathetic efferent, and sensory. The parasympathetic efferent branch is responsible for the accommodation reflex. Damage to this nerve can lead to a loss of accommodation and pupillary constriction, resulting in a dilated pupil that is unresponsive to light.
**Why Each Wrong Option is Incorrect**
* **Option A:** The trigeminal nerve (cranial nerve V) is responsible for sensory innervation of the face, including the cornea and conjunctiva, but it is not involved in the accommodation reflex.
* **Option B:** The facial nerve (cranial nerve VII) controls facial expressions, taste, and some parasympathetic functions, but it is not directly involved in the accommodation reflex.
* **Option C:** The trochlear nerve (cranial nerve IV) is responsible for innervating the superior oblique muscle, which is involved in eye movement, but it is not involved in the accommodation reflex.
**Clinical Pearl / High-Yield Fact**
The oculomotor nerve is the third cranial nerve and is often referred to as the "oculomotor nerve" due to its role in controlling eye movement. Damage to this nerve can result in a range of symptoms, including ptosis (drooping eyelid), diplopia (double vision), and a dilated pupil that is unresponsive to light.
**Correct Answer:** C. The trochlear nerve (cranial nerve IV) is responsible for innervating the superior oblique muscle, which is involved in eye movement, but it is not involved in the accommodation reflex.