What is the primary cause of rigor mortis?

The primary cause of rigor mortis is lactic acid accumulation in the muscles after death. This physiological process begins shortly after a person passes away when cellular metabolism ceases and the body becomes deprived of oxygen. As a result, muscle cells switch to anaerobic metabolism, leading to lactic acid build-up within the cells.

This accumulation of lactic acid lowers the pH within the muscle tissues, resulting in the inability of the muscle fibers to relax. The actin and myosin proteins within the muscle fibers become activated and form cross-links, leading to the stiffening of muscles, which is the hallmark of rigor mortis. Rigor mortis generally begins within a few hours post-mortem and can last for about 24 to 48 hours before the muscles begin to decompose and the stiffness dissipates.

In contrast, factors like temperature levels can influence the onset and duration of rigor mortis but are not the primary cause. Cellular breakdown and enzyme activity occur during decomposition but do not play a crucial role in the immediate onset of rigor mortis. Therefore, lactic acid accumulation directly contributes to the physiological changes observed in rigor mortis, making it the correct response.