Bilateral paralysis of accommodation can occur in patients with:
**Core Concept**
Bilateral paralysis of accommodation is a condition where the muscles responsible for changing the shape of the lens in the eye to focus on near objects are paralyzed. This leads to difficulty in accommodating and focusing on objects at different distances. The ciliary muscles and the sphincter pupillae muscle are involved in this process.
**Why the Correct Answer is Right**
Bilateral paralysis of accommodation is commonly seen in patients with third cranial nerve (oculomotor nerve) palsy. The oculomotor nerve controls the ciliary muscles, which are responsible for changing the shape of the lens. When the oculomotor nerve is paralyzed, the ciliary muscles are unable to contract, leading to paralysis of accommodation. This can result in difficulty in reading, watching TV, or performing other tasks that require focusing on objects at different distances.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because paralysis of accommodation is not typically associated with fourth cranial nerve (trochlear nerve) palsy, which primarily affects the superior oblique muscle.
**Option B:** This option is incorrect because Horner's syndrome, which is caused by disruption of the sympathetic nerves supplying the eye, affects the pupil and eyelid, but not the muscles responsible for accommodation.
**Option C:** This option is incorrect because Marcus Gunn pupil, also known as a relative afferent pupillary defect, is a condition where there is a difference in pupillary response between the two eyes, but it does not affect accommodation.
**Clinical Pearl / High-Yield Fact**
When evaluating a patient with difficulty focusing on objects at different distances, it's essential to examine the extraocular muscles and check for third cranial nerve palsy, which can cause bilateral paralysis of accommodation.
**Correct Answer: C. Third cranial nerve (oculomotor nerve) palsy.**