**Core Concept**
Immediate stiffness postmortem, also known as rigor mortis, is a physiological process that occurs after death due to the cessation of ATP production in muscle cells. This leads to the contraction of actin and myosin filaments, resulting in stiffness and rigidity of the body.

**Why the Correct Answer is Right**
Rigor mortis is caused by the depletion of ATP, which is necessary for the relaxation of muscle cells. Normally, ATP is produced through cellular respiration, but after death, the process of cellular respiration ceases, leading to a rapid decrease in ATP levels. As a result, the actin and myosin filaments in the muscle cells contract, causing stiffness and rigidity. This process typically begins within 2-4 hours after death and peaks at around 12 hours.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the cause of immediate stiffness postmortem.
**Option B:** This option is incorrect because it is not a recognized term related to the process of rigor mortis.
**Option C:** This option is incorrect because it is not a valid explanation for the immediate stiffness postmortem.

**Clinical Pearl / High-Yield Fact**
It's essential to note that rigor mortis can be delayed or absent in certain conditions, such as in cases of prolonged illness or in individuals with certain medical conditions that affect muscle function. Additionally, the process of rigor mortis can be influenced by environmental factors, such as temperature and humidity.

**Correct Answer: D. Postmortem changes due to ATP depletion.**