A patient presented with history of few episodes of vomiting and was given an anti-emetic drug by the intern. Few hours later, the patient developed an abnormal posture. Which of the following is likely drug administered to him?
First, I need to recall the common anti-emetic drugs and their side effects. The abnormal posture suggests some form of extrapyramidal symptoms (EPS), which are movement disorders. These are often associated with dopamine receptor antagonists, especially first-generation ones like metoclopramide or prochlorperazine.
Metoclopramide works by blocking dopamine D2 receptors in the chemoreceptor trigger zone (CTZ) and enhancing acetylcholine activity in the upper GI tract. However, blocking dopamine receptors in the basal ganglia can lead to EPS, such as dystonia, akathisia, or parkinsonism. The abnormal posture here is probably dystonia, which is a common acute EPS.
Now, looking at the options (even though they aren't listed here), the correct answer would be a dopamine antagonist. Other anti-emetics like ondansetron (a 5-HT3 antagonist) don't usually cause EPS. So if the options include metoclopramide, that's the likely culprit. The other options might be drugs that don't block dopamine receptors, so they wouldn't cause this side effect.
The key here is connecting the anti-emetic with the side effect. EPS is a classic sign of dopamine antagonists. The clinical pearl is to remember that first-generation anti-emetics are more prone to causing EPS compared to second-generation ones like ondansetron or granisetron. Also, dystonia typically occurs within hours of administration, which fits the timeline here.
**Core Concept**
This question tests knowledge of extrapyramidal side effects (EPS) associated with dopamine receptor antagonists, particularly first-generation anti-emetics. EPS include dystonia, akathisia, and parkinsonism, caused by dopamine blockade in basal ganglia pathways.
**Why the Correct Answer is Right**
Metoclopramide, a first-generation dopamine D2 receptor antagonist, is known to induce acute dystonia (e.g., oculogyric crisis, torticollis) by blocking dopamine receptors in the nigrostriatal pathway. Its high affinity for D2 receptors in the basal ganglia disrupts motor control, leading to abnormal postures. This side effect typically manifests within hours of administration, aligning with the clinical scenario.
**Why Each Wrong Option is Incorrect**
**Option A:** Ondansetron (5-HT3 antagonist) lacks dopamine-blocking activity and does not cause EPS.
**Option B:** Prochlorperazine, while a dopamine antagonist, is less likely to cause dystonia compared to metoclopramide due to lower peripheral muscarinic effects.
**Option C:** Domperidone has minimal central penetration, making CNS-related EPS rare.
**Clinical Pearl / High-Yield Fact**
First-generation anti-emetics (e.g., metoclopramide, prochlorperazine) are notorious for EPS due to central dopamine blockade. Second-generation agents (e.g., ondansetron) avoid this by lacking dopamine antagonist activity. Always consider EPS when using dopamine-blocking drugs in young patients.
**Correct Answer: C. Metoclopramide**