**Core Concept**
Postmortem caloricity refers to the phenomenon where the body temperature remains elevated or even increases after death, contrary to the expected decrease in temperature. This occurs due to various factors, including the body's metabolic processes, the presence of certain bacteria, and external environmental factors.

**Why the Correct Answer is Right**
Postmortem caloricity can be seen in cases of putrefaction, where bacteria such as Clostridium perfringens and Escherichia coli break down the body's tissues, releasing heat and causing the temperature to rise. This process can also be influenced by the ambient temperature and the presence of other microorganisms. Additionally, certain conditions such as sepsis or severe infections can lead to postmortem caloricity due to the body's heightened metabolic response.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because postmortem caloricity is indeed seen in cases of putrefaction, which involves the breakdown of body tissues by bacteria.
**Option B:** This option is incorrect because postmortem caloricity can be caused by external factors such as ambient temperature, making it a valid reason for its occurrence.
**Option C:** This option is incorrect because postmortem caloricity is not typically seen in cases of hypothermia, where the body temperature is actually decreased.

**Clinical Pearl / High-Yield Fact**
It's essential to note that postmortem caloricity can be a challenging sign to interpret, as it can be influenced by various factors. Therefore, a thorough investigation of the circumstances surrounding the death is necessary to determine the underlying cause of the elevated body temperature.

**Correct Answer: D. Hypothermia.**