**Core Concept**
Lithium is a mood stabilizer used in the treatment of bipolar disorder. However, its therapeutic window is narrow, and toxicity can occur with serum levels above 1.5 mEq/L. Monitoring of lithium levels is crucial to prevent toxicity and ensure optimal therapeutic effects.

**Why the Correct Answer is Right**
When a patient on lithium presents with symptoms of depression, the first step is to check the lithium level. This is because lithium toxicity can masquerade as depressive symptoms, and correcting the toxicity can improve the patient's mood. Elevated lithium levels can cause a range of symptoms, including tremors, ataxia, confusion, and in severe cases, seizures and coma. The psychiatrist should order a lithium level to confirm whether the patient is within the therapeutic range (0.6-1.2 mEq/L) or if toxicity is present.

**Why Each Wrong Option is Incorrect**
**Option A:** Checking the patient's medication adherence is not the immediate next step. While poor adherence can lead to decreased efficacy, it is not the primary concern in this situation.

**Option B:** Starting a new antidepressant is not the correct next step. Antidepressants can interact with lithium and increase the risk of toxicity.

**Option C:** Prescribing a benzodiazepine is not the immediate next step. While benzodiazepines can be used to treat acute agitation or anxiety, they do not address the underlying issue of potential lithium toxicity.

**Clinical Pearl / High-Yield Fact**
Remember the "Lithium Toxicity" mnemonic: "LIT" stands for "Lab work, Inspect the patient, Treat." This reminds you to check the lithium level, inspect the patient for signs of toxicity, and treat accordingly.

**Correct Answer: C. Check the patient's lithium level.**