## Core Concept
The patient's symptoms suggest a side effect profile that includes tremors, polydipsia (excessive thirst), and polyuria (frequent urination). These symptoms are commonly associated with certain psychiatric medications.

## Why the Correct Answer is Right
The correct answer, **D. Lithium**, is a medication commonly used in psychiatry, particularly for treating bipolar disorder. Lithium can cause several side effects that match the patient's symptoms:
- **Tremors**: A fine hand tremor is a common side effect of lithium, occurring in a significant number of patients.
- **Polydipsia and Polyuria**: Lithium can cause nephrogenic diabetes insipidus, leading to increased thirst and urination. This occurs because lithium interferes with the kidneys' ability to concentrate urine.

## Why Each Wrong Option is Incorrect
- **Option A:** While antipsychotics can cause tremors as part of extrapyramidal side effects (EPS), they are less commonly associated with the combination of tremors, polydipsia, and polyuria.
- **Option B:** Antidepressants can have various side effects but are not typically associated with the specific combination of symptoms described.
- **Option C:** While some psychiatric medications can cause anticholinergic side effects (like dry mouth, constipation), the specific constellation of tremors, excessive thirst, and urination points more towards lithium.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is to monitor lithium levels regularly, as the therapeutic window is narrow. Side effects can often be managed by adjusting the dose or adding other medications to mitigate them. Lithium toxicity can present with symptoms similar to those described but also includes nausea, vomiting, ataxia, and confusion at higher levels.

## Correct Answer: D. Lithium.