**Core Concept:** Rigor mortis is a post-mortem change in muscle tone that occurs due to the autolytic action of muscle enzymes. It starts in the muscles and progresses to the joints and skin.

**Why the Correct Answer is Right:** The first site of rigor mortis occurs in the muscles due to the activity of muscle enzymes, particularly the calpains and myosins. These enzymes break down the muscle proteins, causing the characteristic stiffness and rigidity of muscles after death.

**Why Each Wrong Option is Incorrect:**
A. **Option A:** This option is incorrect because rigor mortis starts in the muscles due to enzyme activity.
B. **Option B:** While rigor mortis eventually involves the joints and skin, it does not specifically start in these areas.
C. **Option C:** Rigor mortis begins in the muscles, not the heart, which is not directly related to the process of rigor mortis.
D. **Option D:** Rigor mortis starts in the muscles, not the brain, which is also not directly related to the process of rigor mortis.

**Clinical Pearl:** Rigor mortis is an important factor to consider in forensic medicine, as it can help determine the post-mortem interval or time since death.

**Correct Answer:** Option A (Rigor mortis starts in the muscles).