**Core Concept**
Methysergide is a semi-synthetic ergot alkaloid used primarily for the prophylaxis of migraine headaches. Its mechanism of action involves agonism of serotonin 5-HT1 and 5-HT2 receptors, which leads to vasoconstriction and inhibition of neurotransmitter release.

**Why the Correct Answer is Right**
The characteristic adverse effect associated with methysergide is retroperitoneal fibrosis, a condition characterized by the proliferation of fibrous connective tissue in the retroperitoneum. This is thought to occur due to the ergot alkaloid properties of methysergide, which can cause vasoconstriction and fibrosis in various tissues. The exact mechanism is not fully understood, but it is believed to involve the activation of fibroblasts and the deposition of collagen.

**Why Each Wrong Option is Incorrect**
**Option A:** Nausea and vomiting are common side effects of many medications, including methysergide, but they are not the characteristic adverse effect associated with this drug.
**Option B:** Headache is the primary indication for methysergide, not an adverse effect.
**Option C:** While ergot alkaloids can cause vasoconstriction, this is not the characteristic adverse effect associated with methysergide.

**Clinical Pearl / High-Yield Fact**
Retroperitoneal fibrosis is a rare but serious adverse effect of methysergide, and patients should be monitored for signs and symptoms of this condition, such as abdominal pain and hydronephrosis.

**Correct Answer: C. Fibrosis of the retroperitoneal tissues.**