**Core Concept**
Heat stiffening, also known as heat stroke or heat collapse, is a life-threatening condition that occurs when the body's heat-regulation mechanisms are overloaded, causing a rapid increase in body temperature. This can happen when an individual is exposed to high temperatures, especially in humid environments, without adequate cooling or fluid replacement.

**Why the Correct Answer is Right**
Heat stiffening occurs when the body's temperature exceeds 40.6°C (105°F), causing the hypothalamus to lose its ability to regulate body temperature. The brain's temperature regulation center is damaged, leading to a loss of consciousness, seizures, and eventually death if not treated promptly. The mechanism involves the breakdown of the hypothalamic-pituitary-adrenal axis, leading to an inability to produce sweat and cool the body effectively.

**Why Each Wrong Option is Incorrect**
* **Option A:** 37°C is the normal body temperature, and exposure to this temperature would not cause heat stiffening.
* **Option B:** 42°C is the correct answer, as heat stiffening typically occurs when the body temperature exceeds 40.6°C, but 42°C is often cited as the temperature at which heat stiffening occurs.
* **Option D:** This option is not provided, so we'll skip it.

**Clinical Pearl / High-Yield Fact**
Heat stiffening is a medical emergency that requires prompt recognition and treatment. The classic signs include a body temperature above 40.6°C, confusion, seizures, and a dry, flushed skin. If you suspect heat stiffening, call for medical help immediately and start cooling the patient with cold water or a fan.

**Correct Answer:** B. 42°C