**Question:** Which of the following is the first site of appearance of Rigor mortis after death?

*Core Concept*: Rigor mortis is a series of post-mortem changes in muscle due to the cessation of muscle function and the accumulation of lactic acid. It consists of three stages: early, middle, and late.

**Why the Correct Answer is Right:** The correct answer, **Option C: Muscle** is the first site of appearance of rigor mortis after death. This is because muscle fibers are the primary site where contractile proteins (actin and myosin) are present, and they undergo chemical changes due to the accumulation of lactic acid. As the body cools, the pH decreases, leading to the denaturation of these proteins and the development of rigor mortis.

**Why Each Wrong Option is Incorrect:**

A. **Option A: Liver**: Although liver enzymes are involved in various biochemical reactions, liver is not the first site of appearance of rigor mortis after death.

B. **Option B: Heart**: The heart is responsible for circulatory function, not muscle contraction. Rigor mortis starts in the muscles, not the heart.

D. **Option D: Skin**: Skin is primarily involved in sensory function and does not have significant contractile proteins for rigor mortis to occur.

**Clinical Pearl**: The understanding of rigor mortis is crucial for forensic examination and estimation of post-mortem interval. The progression of rigor mortis can help in determining the time since death occurred.

**Correct Answer:** **Option C: Muscle** is the first site of appearance of rigor mortis after death, primarily due to the accumulation of lactic acid in muscle fibers leading to contractile protein denaturation and muscle rigidity.