**Core Concept**
Haloperidol is a first-generation antipsychotic medication primarily used to treat schizophrenia and other psychoses. Its side effect profile is a significant concern due to its strong dopaminergic blockade.

**Why the Correct Answer is Right**
The most common side effect of haloperidol is **extrapyramidal symptoms (EPS)**, which include dystonia, parkinsonism, and akathisia. This is due to its potent blockade of D2 receptors in the striatum, leading to an imbalance in the dopaminergic neurotransmission. The blockade of D2 receptors disrupts the normal function of the basal ganglia, resulting in the characteristic EPS symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Orthostatic hypotension is a side effect of antihypertensive medications, not typically associated with haloperidol.
**Option B:** Sedation is a common side effect of many antipsychotics, including haloperidol, but it is not the most common side effect.
**Option C:** Weight gain is a side effect of many antipsychotics, including second-generation antipsychotics like olanzapine, but it is not the most common side effect of haloperidol.

**Clinical Pearl / High-Yield Fact**
Haloperidol is a high-potency antipsychotic, meaning it has a low dose required to achieve a therapeutic effect but also a higher risk of EPS side effects.

**Correct Answer: C. Extrapyramidal symptoms**