**Core Concept**
Malignant hyperthermia (MH) is a life-threatening medical emergency triggered by certain volatile anesthetics and depolarizing muscle relaxants. It is characterized by an excessive release of intracellular calcium ions (Ca2+), leading to uncontrolled muscle contractions and hypermetabolism.

**Why the Correct Answer is Right**
The correct answer, **C. Increased intracellular Ca**, is due to the abnormal response of the ryanodine receptor (RyR) to volatile anesthetics and depolarizing muscle relaxants. In patients with MH susceptibility, the RyR is hypersensitive to these agents, leading to an excessive release of intracellular calcium. This triggers a cascade of events, including muscle contraction, increased metabolism, and hyperthermia. The abnormal calcium release is mediated by the abnormal function of the RyR, which is a voltage-gated calcium channel located on the sarcoplasmic reticulum.

**Why Each Wrong Option is Incorrect**
* **Option A:** Increased intracellular Na is not directly related to malignant hyperthermia. While changes in sodium levels can affect muscle function, the primary mechanism of MH involves an increase in intracellular calcium.
* **Option B:** Decreased intracellular chlorine is not a known mechanism of malignant hyperthermia. Chloride channels are involved in various physiological processes, but their dysfunction is not directly linked to MH.
* **Option D:** Increased serum K is not a direct cause of malignant hyperthermia. While hyperkalemia can occur in MH due to muscle cell breakdown, it is a secondary effect rather than the primary mechanism.

**Clinical Pearl / High-Yield Fact**
Malignant hyperthermia is a medical emergency that requires immediate treatment with dantrolene, a specific antagonist of the RyR. Early recognition and treatment are critical to prevent long-term complications and mortality.

**✓ Correct Answer: C. Increased intracellular Ca. Increased intracellular calcium ions trigger malignant hyperthermia through abnormal ryanodine receptor function.**