Explanation of the state of rigor mortis

A 66-year-old man who lives alone has a severe myocardial infarction and dies during the night. The medical examiner’s office is called the following morning and describes the man’s body as being in rigor mortis. The state of rigor mortis is due to:

• a.Inhibition of Ca++ leakage from the extracellular fluid and sarcoplasmic reticulum
• b.Enhanced retrieval of calcium by the sarcoplasmic reticulum
• c.Failure to disengage tropomyosin and troponin from the myosin active sites
• d.Absence of ATP preventing detachment of the myosin heads from actin
• e.Increased lactic acid production

The answer is d. 

There is some small amount of production of ATP after death through anaerobic and phosphagen pathways. However, there is insufficient ATP to induce the detachment of the myosin heads from actin. Ca ions continue to leak from the extracellular fluid and the sarcoplasmic reticulum (answer a),
however, the sarcoplasmic reticulum is no longer able to retrieve the Ca ions (answer b). Tropomyosin and troponin are disengaged from the myosin active sites (answer c). Lactic acid is produced during rigor mortis through anaerobic pathways. The high levels of lactic acid cause deterioration of the skeletal muscle and end the state of rigor mortis (answer e).