**Core Concept**
Methysergide is a semisynthetic ergot alkaloid used in the prophylactic treatment of migraine and cluster headaches. Its mechanism of action involves agonism at serotonin receptors, particularly 5-HT1 and 5-HT2 receptors. However, methysergide's clinical use is limited due to its potential for causing fibrotic reactions.

**Why the Correct Answer is Right**
Methysergide can cause fibrotic reactions, including pulmonary fibrosis, which is a well-documented side effect of this medication. This occurs due to the ergot alkaloid's ability to stimulate fibroblast proliferation and collagen synthesis, leading to the development of fibrotic lesions in various organs, including the lungs. The incidence of pulmonary fibrosis associated with methysergide is higher in patients taking the medication for extended periods.

**Why Each Wrong Option is Incorrect**
**Option B:** Pleural effusion is not a known side effect of methysergide, although it may occur in patients with underlying respiratory conditions or as a result of other medications.

**Option C:** Syncope is not a direct consequence of methysergide therapy, although patients with migraines may experience syncope due to other factors such as vasovagal reactions.

**Option D:** Myocarditis is not a recognized side effect of methysergide, although ergot alkaloids can cause vasoconstriction and potentially lead to cardiovascular complications in rare cases.

**Clinical Pearl / High-Yield Fact**
Methysergide is contraindicated in patients with a history of fibrotic reactions, and its use should be carefully monitored in patients with underlying respiratory or cardiovascular conditions. A thorough medical history and regular follow-up are essential to minimize the risk of adverse effects.

**✓ Correct Answer: A. Pulmonary fibrosis**