## Core Concept
The question pertains to the phenomenon of **cadaveric rigidity**, also known as rigor mortis, and its prolongation in cases of poisoning. Rigor mortis is a well-known post-mortem change that occurs in the muscles of a deceased individual, causing temporary stiffening of the body. Various factors, including environmental conditions and certain chemicals, can affect the duration and intensity of rigor mortis.

## Why the Correct Answer is Right
The correct answer, **D. Arsenic**, is associated with a prolongation of rigor mortis. Arsenic is known to preserve tissues and can cause a delay in the onset and prolongation of rigor mortis. This effect is due to its **inhibitory action on ATPase**, an enzyme crucial for muscle relaxation during the process of rigor mortis resolution. As a result, in cases of arsenic poisoning, the body may remain stiff for a longer period than usual.

## Why Each Wrong Option is Incorrect
- **Option A:** **Strychnine** causes muscle convulsions and stiffness due to its action as a **glycine receptor antagonist** in the spinal cord. However, it does not typically prolong rigor mortis but rather causes death through convulsions.
- **Option B:** **Morphine** is an opioid that can cause **respiratory depression** leading to death. While it affects various physiological processes, it does not have a direct effect on the duration of rigor mortis.
- **Option C:** **Cyanide** acts by inhibiting **cytochrome c oxidase** in mitochondria, causing cellular asphyxiation. It leads to rapid death but does not specifically prolong rigor mortis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **arsenic poisoning** can lead to a prolongation of **rigor mortis**, making it a potential clue in forensic investigations. This association can be critical in the context of forensic medicine and toxicology.

## Correct Answer: D. Arsenic