A patient with acute psychosis, who is on haloperidol 20mg/day for last 2 days, has an episode characterized by tongue protrusion, oculogyric crisis, stiffness and abnormal posture of limbs and trunk without loss of consciousness for last 20 minutes before presenting to casualty. This improved within a few minutes after administration of diphenhydramine HCl. The most likely diagnosis is

**Core Concept**
Haloperidol, a typical antipsychotic, can cause a range of extrapyramidal side effects (EPS) due to its blockade of dopamine D2 receptors in the basal ganglia. This leads to an imbalance in the dopaminergic and cholinergic systems, causing dystonic reactions among other symptoms.

**Why the Correct Answer is Right**
The symptoms described, such as tongue protrusion, oculogyric crisis, stiffness, and abnormal posture of limbs and trunk, are characteristic of a dystonic reaction, a type of EPS. Haloperidol's blockade of dopamine D2 receptors in the striatum disrupts the normal balance between dopaminergic and cholinergic neurotransmission. Diphenhydramine, an antihistamine with anticholinergic properties, is often used to treat dystonic reactions by counteracting the excess cholinergic activity. The rapid improvement after diphenhydramine administration supports this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a correct answer as it is not provided.
**Option B:** Other options are not provided for evaluation.
**Option C:** Not a correct answer as it is not provided.
**Option D:** Not a correct answer as it is not provided.

**Clinical Pearl / High-Yield Fact**
Dystonic reactions can be managed with anticholinergic agents like diphenhydramine or benztropine, but it's essential to recognize that these reactions are a sign of underlying EPS, which may require dose adjustment or switching to an atypical antipsychotic.

**Correct Answer:** D.

✓ Correct Answer: A. Acute dystonia