## **Core Concept**
The question tests knowledge of side effects associated with psychotropic medications, specifically those that can cause a combination of tremor, thyroid enlargement, and leucocytosis. This constellation of symptoms suggests a drug that affects multiple systems, including the nervous, endocrine, and hematologic systems.

## **Why the Correct Answer is Right**
Lithium is well-known for causing a range of side effects, including **tremors** (a common neurological side effect), **thyroid enlargement** (due to its effect on thyroid function, leading to hypothyroidism in some cases), and **leucocytosis** (an increase in white blood cell count). These side effects are classic and dose-related, making lithium a prime suspect in this clinical scenario.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While anticholinergic side effects or extrapyramidal side effects (like tremors) can occur with various psychotropic drugs, the specific combination of tremor, thyroid enlargement, and leucocytosis is less typical for drugs primarily causing anticholinergic or dopaminergic blockade effects.
- **Option B:** Atypical antipsychotics can cause a variety of side effects, including metabolic effects and extrapyramidal symptoms, but the specific combination mentioned is not as uniquely characteristic as it is for lithium.
- **Option C:** This option does not provide a specific drug to evaluate, making it impossible to assess its relevance directly.
- **Option D:** Similar to option A, without a specific drug, it's hard to directly refute, but generally, the side effect profile of many antidepressants doesn't typically include thyroid enlargement and significant leucocytosis as common side effects.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is to monitor lithium levels regularly, as its therapeutic window is narrow. Side effects can often be managed by adjusting the dose. The combination of tremor, thyroid changes, and leucocytosis should prompt consideration of lithium as a causative agent in psychiatric patients.

## **Correct Answer: C. Lithium.**