**Core Concept**
Rigor mortis is a temporary stiffening of the muscles that occurs after death, caused by the breakdown of ATP and the resulting accumulation of lactic acid, leading to an increase in intracellular calcium and the activation of the contractile apparatus in skeletal muscle cells.

**Why the Correct Answer is Right**
The correct answer is related to the concept that rigor mortis is influenced by the amount of ATP available in the muscles at the time of death. When ATP is depleted, the muscles enter a state of rigor mortis, characterized by a rigid and stiffened appearance. This process is more prolonged in situations where there is a delay in ATP depletion, such as in hypothermic or poisoned individuals.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly relate to the biochemical process underlying rigor mortis.
**Option B:** This option is incorrect because it is not a factor that influences the duration of rigor mortis.
**Option C:** This option is incorrect because it is not a direct cause of longer rigor mortis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that rigor mortis can be delayed or absent in cases of hypothermia, severe poisoning, or in individuals who have undergone prolonged physical activity before death, as these conditions can lead to a depletion of ATP stores.

**Correct Answer:** D.