**Core Concept**
Neuroleptic malignant syndrome (NMS) is a life-threatening neurological disorder associated with the use of antipsychotic medications, characterized by fever, muscle rigidity, altered mental status, and autonomic dysfunction.

**Why the Correct Answer is Right**
The symptom of NMS is hyperthermia, which is a result of the body's inability to regulate its temperature. This is due to the blockade of dopamine receptors in the hypothalamus, leading to an imbalance in the body's thermoregulatory mechanisms. The blockade of dopamine receptors also contributes to the development of muscle rigidity, which further exacerbates the hyperthermia.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypotension is not a symptom of NMS. In fact, patients with NMS often exhibit hypertension due to autonomic dysfunction.
**Option B:** Seizures are not a characteristic symptom of NMS, although they can occur in some cases. The primary symptoms of NMS include fever, muscle rigidity, altered mental status, and autonomic dysfunction.
**Option C:** Diarrhea is not a symptom of NMS. Gastrointestinal symptoms such as constipation or abdominal pain may occur, but diarrhea is not a typical feature of this condition.

**Clinical Pearl / High-Yield Fact**
NMS is a medical emergency that requires immediate recognition and treatment. The diagnosis of NMS is based on clinical criteria, including fever, muscle rigidity, altered mental status, and autonomic dysfunction. A high index of suspicion is essential for early diagnosis and treatment.

**Correct Answer: D. Hyperthermia.**