**Core Concept**
Methysergide is a **serotonin antagonist** used for prophylactic treatment of migraine and cluster headaches. It acts by blocking **5-HT2 receptors**, which are involved in the pathogenesis of these conditions.

**Why the Correct Answer is Right**
The correct answer is associated with the unique side effect profile of methysergide, which includes **retroperitoneal fibrosis**. This is a rare but serious condition characterized by the formation of fibrotic tissue in the retroperitoneum, potentially leading to bowel or ureteral obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the characteristic adverse effect of methysergide.
**Option B:** Similarly, this option is incorrect as it is not specifically associated with methysergide.
**Option C:** This option is also incorrect, lacking a direct connection to methysergide's side effect profile.
**Option D:** This option is not the correct answer, as the characteristic adverse effect of methysergide is not listed here.

**Clinical Pearl / High-Yield Fact**
Methysergide is known for its rare but serious side effect of **retroperitoneal fibrosis**, which is a key point to remember for exams.

**Correct Answer:** D. Retroperitoneal fibrosis.