**Core Concept**
Neuroleptic malignant syndrome (NMS) is a life-threatening neurological disorder caused by an adverse reaction to neuroleptic or antipsychotic medication. It is characterized by a combination of symptoms including fever, altered mental status, and autonomic dysfunction.

**Why the Correct Answer is Right**
Labile hypertension, which refers to sudden and unpredictable fluctuations in blood pressure, is a hallmark feature of NMS. This is due to the intense sympathetic overactivity associated with the condition, which leads to excessive release of catecholamines such as adrenaline and noradrenaline. As a result, patients with NMS often exhibit rapid and unpredictable changes in blood pressure, ranging from hypertension to hypotension.

**Why Each Wrong Option is Incorrect**
**Option A:** Bradycardia is not typically associated with NMS. In fact, tachycardia is a more common finding in these patients due to the sympathetic overactivity.
**Option C:** Hypotonia, or decreased muscle tone, is not a characteristic feature of NMS. Patients with NMS often exhibit increased muscle tone, rigidity, and stiffness.
**Option D:** Hypothermia is not a typical feature of NMS. In fact, fever is a common finding in these patients, often exceeding 38°C.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of NMS is the presence of muscle rigidity, which can be so severe that it leads to rhabdomyolysis and acute kidney injury. Early recognition and treatment of NMS are critical to prevent these complications.

✓ Correct Answer: B. Labile hypertension