What is rigor mortis?

Rigor mortis is a physiological phenomenon that occurs after death, characterized by the stiffening of the body's muscles and joints. This process typically begins within a few hours after death due to biochemical changes that happen in muscle tissue as a result of the depletion of adenosine triphosphate (ATP). Without ATP, the muscles cannot relax, leading to rigidity. Rigor mortis is one of the key signs that forensic scientists and medical examiners assess to help determine the time of death, as its onset and duration can provide crucial information.

The other options focus on different aspects of post-mortem changes. The first option pertains to brain death, which is a medical definition of death concerning brain function but does not directly relate to the physical state of the body after death. The third option, which describes flatulence in the deceased, is an unrelated bodily function that may occur due to decomposition processes but does not pertain to muscle stiffness. The last option refers to post-mortem body cooling, known as algor mortis, which is a separate phenomenon from rigor mortis, as it deals with the dropping body temperature rather than muscle rigidity. Thus, the correct answer accurately captures the essence of rigor mortis.