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 HC1. The most likely diagnosis is:

**Core Concept**
Acute psychosis in a patient on haloperidol is associated with the risk of extrapyramidal side effects (EPS), including dystonic reactions, which are characterized by involuntary muscle contractions leading to abnormal postures and movements.

**Why the Correct Answer is Right**
The patient's symptoms of tongue protrusion, oculogyric crisis, stiffness, and abnormal posture of limbs and trunk without loss of consciousness are classic signs of a dystonic reaction, a type of EPS. Haloperidol, a typical antipsychotic, is a dopamine D2 receptor antagonist that can cause EPS. Diphenhydramine, an antihistamine with anticholinergic properties, is often used to treat EPS by blocking the action of acetylcholine, which can help alleviate dystonic reactions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because akathisia, a type of EPS characterized by restlessness and a feeling of inner tension, typically does not present with the specific symptoms described in the patient.
**Option B:** This option is incorrect because parkinsonism, a type of EPS characterized by tremors, rigidity, and bradykinesia, typically does not present with the acute onset of symptoms described in the patient.
**Option C:** This option is incorrect because tardive dyskinesia, a type of EPS characterized by involuntary movements, typically develops after long-term use of antipsychotics and is not associated with the acute onset of symptoms described in the patient.

**Clinical Pearl / High-Yield Fact**
Diphenhydramine is often used to treat EPS, including dystonic reactions, due to its anticholinergic properties, which can help alleviate symptoms by blocking the action of acetylcholine.

**Correct Answer:** A. Dystonic reaction.

✓ Correct Answer: A. Acute dystonia