## Core Concept
Rigor mortis is a well-known postmortem change that occurs in the muscles of the deceased, leading to temporary stiffening of the body. It is a result of changes in muscle physiology after death, specifically related to the depletion of ATP and alterations in muscle protein interactions. The process involves the cross-bridging of actin and myosin filaments in muscle cells.

## Why the Correct Answer is Right
The correct answer, **D. Small muscles of the face**, indicates that rigor mortis first occurs in the smaller muscles, particularly those of the face. This happens because smaller muscles have a higher surface-to-volume ratio, which affects the rate of postmortem changes, including the onset of rigor mortis. Additionally, muscles with higher metabolic rates tend to undergo rigor mortis earlier. The small muscles of the face, being highly specialized and having a relatively high metabolic rate, are among the first to exhibit rigor mortis.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although muscles in the lower limbs do undergo rigor mortis, they are not typically the first to be affected due to their size and metabolic rate compared to smaller muscles.
- **Option B:** This option is incorrect as it does not specify a particular muscle group; hence, it cannot be evaluated directly.
- **Option C:** This option is incorrect because, similar to option A, it does not accurately represent the muscle group that is first affected by rigor mortis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that rigor mortis usually begins to set in around 2 hours after death and can last for up to 18-36 hours, depending on various factors like ambient temperature. The process starts in smaller muscles and gradually involves larger muscle groups.

## Correct Answer: D. Small muscles of the face