**Core Concept**
Rigor mortis is a post-mortem process characterized by the stiffening of the body's muscles due to changes in the muscle's protein structure. It typically begins within 2-4 hours after death and peaks around 12 hours, gradually resolving after 24-48 hours. The onset and duration of rigor mortis are influenced by factors such as body temperature, physical condition of the individual, and environmental conditions.

**Why the Correct Answer is Right**
The correct answer is **A. Fetus < 7 months of age** because the fetus's muscle structure is still in a state of development and does not contain the same amount of myosin and actin proteins as adult muscles. As a result, the fetus's muscles do not undergo the same changes that lead to rigor mortis. Additionally, the fetus's body temperature is typically higher than that of the adult, which can also influence the onset and duration of rigor mortis.

**Why Each Wrong Option is Incorrect**
**Option B:** Full term fetus - While the fetus's muscle structure is still developing, a full-term fetus is more likely to exhibit some degree of rigor mortis than a fetus less than 7 months old.

**Option C:** Elderly - Rigor mortis can occur in individuals of any age, including the elderly. The onset and duration may be influenced by the individual's physical condition, but it is not a factor that would prevent rigor mortis from occurring.

**Option D:** Anemic person - Anemia may affect the body's ability to recover from rigor mortis, but it does not prevent the onset of rigor mortis.

**Clinical Pearl / High-Yield Fact**
Rigor mortis can be used as an indicator of the post-mortem interval, but it is not a reliable method for estimating time since death, especially in cases where the body has been exposed to extreme temperatures or has undergone significant trauma.

**✓ Correct Answer: A. Fetus < 7 months of age**