**Core Concept**
Postmortem stiffness, also known as rigor mortis, is a temporary condition that occurs after death due to the biochemical changes in the muscle cells. It is a result of the energy depletion in the muscles, leading to an imbalance between actin and myosin filaments, causing muscle rigidity.

**Why the Correct Answer is Right**
Rigor mortis occurs due to the depletion of ATP (adenosine triphosphate) in the muscle cells, which is necessary for the sliding filament theory to occur. The actin and myosin filaments, which are responsible for muscle contraction, become locked in a rigid position, leading to stiffness. This process typically begins within 2-4 hours after death and reaches its peak within 12 hours.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the biochemical changes that cause rigor mortis. It is a distractor that might be related to other postmortem changes.
**Option B:** This option is incorrect because it refers to a different process, such as putrefaction, which is the breakdown of tissues by bacteria and enzymes. While putrefaction can cause changes in the body, it is not responsible for immediate postmortem stiffness.
**Option C:** This option is incorrect because it refers to a medical condition, such as hypokalemia, which can cause muscle weakness but is not directly related to the biochemical changes that cause rigor mortis.

**Clinical Pearl / High-Yield Fact**
Rigor mortis typically begins in the small muscles of the face and jaw, followed by the larger muscles of the body. This can be used as a rough estimate of the time of death.

**Correct Answer: D. Hypostasis (or livor mortis) is incorrect, Hypostasis is the discoloration of the skin due to pooling of blood in the veins after death. The correct answer is actually D. but a better option would have been D. Algor mortis or D. Putrefaction is incorrect but Algor mortis is the cooling of the body after death.