**Core Concept**
Ophthalmoplegic migraine is a rare, severe variant of migraine characterized by unilateral, progressive paralysis of cranial nerves controlling eye movement, most commonly involving the oculomotor nerve. This condition results from vascular ischemia or inflammation affecting the nerve roots, leading to impaired pupillary and extraocular muscle function.

**Why the Correct Answer is Right**
The oculomotor nerve (Cranial Nerve III) is the most frequently affected in ophthalmoplegic migraine. It innervates most extraocular muscles (e.g., superior, inferior, medial rectus, and inferior oblique), the dilator pupillae, and the ciliary body. During attacks, transient ischemia or inflammation in the nerve or its root entry zone causes paralysis, leading to ptosis, diplopia, and impaired eye movement. This pattern is pathognomonic and distinguishes ophthalmoplegic migraine from other neurological disorders.

**Why Each Wrong Option is Incorrect**
Option A: The optic nerve (II) is involved in vision transmission, not eye movement or motor control. It is not affected in ophthalmoplegic migraine.
Option C: The trigeminal nerve (V) is involved in facial sensation and pain, not eye movement. Migraine aura may involve it, but not ophthalmoplegia.
Option D: The abducens nerve (VI) is rare in ophthalmoplegic migraine and typically presents with horizontal gaze palsy, which is less common and less specific.

**Clinical Pearl / High-Yield Fact**
Oculomotor nerve palsy is the hallmark of ophthalmoplegic migraine; it usually occurs in women, during or after a migraine headache, and resolves within hours to days. Always consider this diagnosis in patients with sudden, painless ptosis and diplopia, especially with a history of migraine.

✓ Correct Answer: B. III nerve