**Core Concept**
Instantaneous rigor, also known as instantaneous rigor mortis, is a phenomenon observed in the early stages of death, typically within minutes of demise. It is characterized by the rapid onset of stiffness in the muscles, which is different from the typical rigor mortis that develops several hours post-mortem.

**Why the Correct Answer is Right**
Instantaneous rigor is thought to be caused by the sudden interruption of ATP production in muscle cells, leading to a rapid depletion of energy stores. This results in the contraction of sarcomeres and the subsequent stiffness. The exact mechanisms behind instantaneous rigor are not fully understood, but it is believed to be related to the rapid loss of ATP and the subsequent activation of the myosin light chain kinase (MLCK) pathway.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because instantaneous rigor is not related to the release of histamine, which is more commonly associated with anaphylaxis and vasodilation.
**Option B:** This option is incorrect because instantaneous rigor is not a result of the accumulation of lactic acid, which is more commonly associated with muscle fatigue and cramping.
**Option C:** This option is incorrect because instantaneous rigor is not a result of the activation of the sympathetic nervous system, which is more commonly associated with the fight-or-flight response.

**Clinical Pearl / High-Yield Fact**
Instantaneous rigor is a rare and poorly understood phenomenon that can provide valuable information in forensic medicine, particularly in cases of sudden and unexpected death.

**Correct Answer: D.**