Correct Answer: A. Metoclopramide
Metoclopramide is a dopamine antagonist (D2 receptor blocker) used as an antiemetic in chemotherapy patients. The triad of acute dystonia, bradykinesia, and tremors represents acute extrapyramidal side effects (EPS) — a hallmark of dopamine blockade in the basal ganglia. Metoclopramide crosses the blood-brain barrier and blocks dopamine receptors in the striatum, leading to relative cholinergic excess. This manifests as acute dystonia (involuntary muscle contractions, oculogyric crisis), bradykinesia (slow movement), and tremor. These are dose- and duration-dependent adverse effects, particularly in young patients and those receiving high doses. In Indian clinical practice, metoclopramide is widely used for chemotherapy-induced nausea and vomiting (CINV), but EPS risk necessitates co-prescription of anticholinergics (benztropine) or preference for 5-HT3 antagonists in high-risk patients. The symptom onset is typically acute (within hours to days), distinguishing it from tardive dyskinesia which develops after prolonged exposure. Recognition of this complication is critical because it is reversible with anticholinergic agents or dopamine agonists.
Why the other options are wrong
B. Meclizine — Meclizine is a first-generation antihistamine (H1 blocker) with anticholinergic properties. While it can cause sedation and dry mouth, it does NOT block dopamine and therefore cannot produce acute extrapyramidal symptoms like dystonia, bradykinesia, or tremor. Its mechanism (histamine antagonism) is fundamentally different from metoclopramide's dopamine blockade. NBE may include this to test understanding of antihistamine vs. dopamine antagonist mechanisms. C. Ondansetron — Ondansetron is a 5-HT3 receptor antagonist (selective serotonin blocker) with no dopamine antagonism. It is the preferred antiemetic in modern CINV protocols precisely because it lacks extrapyramidal side effects. While it may cause headache or constipation, it does not produce dystonia, bradykinesia, or tremor. This is a common distractor because both are used as antiemetics in chemotherapy. D. Scopolamine — Scopolamine is an anticholinergic agent (muscarinic antagonist) used for motion sickness and CINV. It causes anticholinergic effects (dry mouth, mydriasis, tachycardia), not dopamine blockade. In fact, anticholinergics are used to treat metoclopramide-induced EPS, not cause them. This option tests whether students confuse anticholinergic toxicity with dopaminergic toxicity.
High-Yield Facts
- Metoclopramide EPS: Acute dystonia, bradykinesia, tremor, and oculogyric crisis occur due to D2 dopamine receptor blockade in the basal ganglia.
- Risk factors for metoclopramide EPS: Young age (<30 years), female sex, high doses (>30 mg/day), and prolonged use increase risk in Indian patient populations.
- Reversibility: Metoclopramide-induced EPS are completely reversible within minutes to hours with IV benztropine (1–2 mg) or diphenhydramine, unlike tardive dyskinesia.
- 5-HT3 antagonists (ondansetron, granisetron) are now preferred for CINV in Indian oncology guidelines because they lack dopamine antagonism and EPS risk.
- Anticholinergic co-prescription: In India, metoclopramide is often given with benztropine prophylactically in high-risk patients to prevent EPS during chemotherapy.
Mnemonics
DOPA-BLOCK = EPS Dopamine Opposition in Putamen And striatum → Bradykinesia, Loss of control (dystonia), Oculogyric crisis, Contraction (tremor), K = extrapyramidal. When dopamine is blocked in basal ganglia, you get EPS. Metoclopramide blocks dopamine → EPS. METRO-EPS (Memory Hook) METROpramide → Extrapyramidal Problem Syndrome. Think of metoclopramide as the 'metro' drug that takes you to EPS station. Use this when you see acute dystonia + antiemetic in chemotherapy.
NBE Trap
NBE pairs metoclopramide with other antiemetics (ondansetron, meclizine, scopolamine) to test whether students confuse mechanism of action. The trap is assuming all antiemetics work similarly — in reality, only dopamine antagonists cause EPS. Students who memorize "metoclopramide = antiemetic" without understanding its dopamine blockade mechanism will struggle.
Clinical Pearl
In Indian oncology wards, when a chemotherapy patient develops acute dystonia or oculogyric crisis after metoclopramide, IV benztropine 1–2 mg is the immediate rescue — symptoms resolve within 5–10 minutes. This rapid reversal is pathognomonic for dopamine antagonist toxicity and helps clinicians distinguish it from other neurological complications of chemotherapy.
_Reference: KD Tripathi Pharmacology Ch. 16 (Antiemetics); Harrison Ch. 297 (Nausea and Vomiting)_