This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Death and Investigations, Subject name: Forensic Medicine
Rigor mortis occurs due to? 
Select and explain. 
A. Breakdown of enzymes in the body, B. Depletion of ATP, C. Accumulation ofblood in dependent areas, D. Loss of heat
B: Depletion of ATP
Answer: B: Depletion of ATP 
Explanation: Ans. is 'b' i.e., Depletion of ATP * Changes in muscles after death are in a sequence of: (i) primary relaxation or flaccidity, (ii) rigor mortis, and (iii) secondary relaxation or flaccidity.* Primary relaxation occurs during somatic death and lasts for 1-2 hours. All the muscles of body begin to relax soon after death. It is due to stoppage of aerobic processes. Muscles remain relaxed as long as the ATP content is sufficient high to permit the splitting of actin-myosin cross bridge.* Once ATP content is completely depleted, stiffening of muscles occur, i.e. Rigor mortis.* Following rigor mortis, muscles become flaccid again due to action of alkaline liquid produced by putrefaction, i.e. secondary relaxation.