**Core Concept**
Clozapine is an atypical antipsychotic medication primarily used to treat schizophrenia, particularly in patients who have not responded to other treatments. Its side effect profile is distinct from typical antipsychotics and includes a range of potential complications.

**Why the Correct Answer is Right**
Clozapine is known for its risk of agranulocytosis, a potentially life-threatening reduction in white blood cells. This occurs due to its effect on the bone marrow, leading to a decrease in neutrophil production. Additionally, clozapine can cause sedation, weight gain, and increased risk of diabetes mellitus. However, it is not typically associated with QT interval prolongation, a condition that can increase the risk of arrhythmias.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as clozapine is known to cause sedation, which is a common side effect of this medication.
**Option B:** Incorrect, as clozapine is associated with weight gain, which is a well-documented side effect.
**Option C:** Incorrect, as clozapine can increase the risk of diabetes mellitus, particularly in patients with pre-existing risk factors.

**Clinical Pearl / High-Yield Fact**
A key consideration when prescribing clozapine is the need for regular monitoring of white blood cell counts to mitigate the risk of agranulocytosis.

**Correct Answer:** C. QT interval prolongation.