**Core Concept**
Succinylcholine-induced masseter muscle spasm, also known as malignant hyperthermia (MH), is a life-threatening complication that occurs in susceptible individuals. It is characterized by muscle rigidity, increased end-tidal CO2 (ETCO2), hyperthermia, and tachycardia.

**Why the Correct Answer is Right**
Malignant hyperthermia is a hypermetabolic state triggered by depolarizing muscle relaxants like succinylcholine. The condition is caused by an abnormality in the ryanodine receptor (RyR1) or the dihydropyridine receptor (DHPR) on skeletal muscle cells, leading to an uncontrolled release of calcium ions. This results in muscle rigidity, increased ETCO2, and a subsequent rise in body temperature.

**Why Each Wrong Option is Incorrect**
* **Option A:** Administration of dantrolene is the primary treatment for malignant hyperthermia. Dantrolene works by inhibiting calcium ion release from the sarcoplasmic reticulum, thereby reversing the hypermetabolic state.
* **Option B:** Cooling the patient with external measures like fans, ice packs, or cool IV fluids may help reduce body temperature but is not a definitive treatment. Dantrolene is the only specific antidote for malignant hyperthermia.
* **Option C:** Halothane and other volatile anesthetics can trigger malignant hyperthermia in susceptible individuals, but they are not part of the management plan. In fact, these agents should be avoided in patients with a history of MH.
* **Option D:** Fentanyl and other opioids can be used to help manage pain and anxiety in patients with malignant hyperthermia, but they do not treat the underlying condition.

**Clinical Pearl / High-Yield Fact**
Malignant hyperthermia can be triggered by various factors, including succinylcholine, volatile anesthetics, and certain genetic mutations. It is essential to have a high index of suspicion for MH in patients who develop muscle rigidity, increased ETCO2, and tachycardia during anesthesia.

**Correct Answer:** D. Fentanyl and other opioids can be used to help manage pain and anxiety in patients with malignant hyperthermia, but they do not treat the underlying condition.