What is rigor mortis?

Rigor mortis refers specifically to the phenomenon where the muscles of a deceased person stiffen due to biochemical changes in the body after death. This process typically begins a few hours after death and can last for several days, depending on factors such as the environment and the physical condition of the body prior to death. The stiffening results from the depletion of adenosine triphosphate (ATP), which is essential for muscle relaxation, causing the muscle fibers to become rigid.

The other options represent distinct post-mortem processes: pooling of blood, known as livor mortis, occurs as gravity causes blood to settle in the lowest parts of the body; the body temperature might rise or fall after death, referred to as algor mortis; and the presence of insects is an indicator of decomposition but does not correlate to the physiological changes of rigor mortis. Thus, rigor mortis specifically signifies the stiffening of muscles, accurately matching the choice identified.