**Core Concept**
Post-mortem rigidity, also known as rigor mortis, is the stiffening of the body after death due to the biochemical changes in the muscles. This phenomenon is caused by the depletion of ATP (adenosine triphosphate) in the muscle cells, leading to the breakdown of the actin and myosin filaments and the subsequent contraction of the muscles.

**Why the Correct Answer is Right**
The correct answer to this question is related to the biochemical process that occurs in the muscles after death. ATP is necessary for the relaxation of muscles, and its depletion leads to the contraction of the muscles, resulting in rigidity. The correct answer is the first to disappear in the sequence of muscle contraction and relaxation, which is essential for understanding the pathophysiology of post-mortem rigidity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the correct answer is related to the biochemical process, not the physical appearance or the time frame of post-mortem changes.
* **Option B:** This option is incorrect because the correct answer is the first to disappear in the sequence of muscle contraction and relaxation, not the last.
* **Option C:** This option is incorrect because it is not directly related to the biochemical process of post-mortem rigidity.
* **Option D:** This option is incorrect because it is not the first to disappear in the sequence of muscle contraction and relaxation.

**Clinical Pearl / High-Yield Fact**
The time frame for post-mortem rigidity is typically 2-4 hours after death, and it is usually at its peak between 2-6 hours. This knowledge is essential for forensic medical examiners to estimate the time of death.

**Correct Answer: D. ATP**