## Core Concept
The question revolves around a common side effect of antipsychotic medications, specifically **haloperidol**, which is a typical antipsychotic used in the management of **schizophrenia**. The symptoms described suggest **akathisia**, a type of extrapyramidal side effect (EPS) characterized by subjective feelings of restlessness and an urge to move, often manifesting as fidgetiness, irritability, and an inability to sit still.

## Why the Correct Answer is Right
Akathisia is a well-known side effect of antipsychotic drugs, particularly first-generation (typical) antipsychotics like **haloperidol**. It is part of the broader category of EPS, which also includes **parkinsonism**, **dystonia**, and **tardive dyskinesia**. The most appropriate treatment strategy for akathisia involves either reducing the dose of the antipsychotic medication, switching to a different antipsychotic with a lower potency or one that is less likely to cause EPS (such as an atypical antipsychotic), or adding a medication to counteract the side effect. **Beta-blockers**, such as **propranolol**, are commonly used off-label to treat akathisia.

## Why Each Wrong Option is Incorrect
- **Option A:** While changing to an atypical antipsychotic could be beneficial in reducing EPS, the question specifically asks for the most appropriate immediate treatment strategy for the described side effect.
- **Option B:** Increasing the dose of haloperidol would likely exacerbate the EPS, including akathisia, making it an inappropriate choice.
- **Option C:** Although adding an anticholinergic might help with some EPS like parkinsonism or dystonia, it is not typically effective for akathisia and could potentially worsen the situation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **akathisia** can be misdiagnosed as **agitation** or **anxiety**, leading to inappropriate treatment with increased antipsychotic dosing or addition of anxiolytics, which can worsen the condition. Recognizing akathisia as a side effect of antipsychotic medication and treating it with **beta-blockers** like propranolol is a valuable clinical strategy.

## Correct Answer Line
**Correct Answer: D.**