**Core Concept**
Rigor mortis is the stiffening of muscles after death due to energy depletion and the subsequent action of calcium ions. This process is influenced by various factors, including the age of the individual, the environment, and the presence of certain medical conditions.

**Why the Correct Answer is Right**
Rigor mortis typically begins to set in 2-4 hours after death in adults and can last for several hours. However, in fetuses, the process is significantly delayed due to the presence of a high concentration of glycogen in the muscles, which serves as an energy source. As a result, rigor mortis is usually not seen before 7 months of gestation, as the fetal muscles require a longer period to deplete their glycogen stores. This delayed onset of rigor mortis in fetuses can be a useful indicator in forensic investigations to estimate the post-mortem interval.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because rigor mortis can be seen in both upper and lower limbs in fetuses, just as in adults. The distribution of rigor mortis is not limited to specific limbs.
**Option C:** This option is incorrect because rigor mortis can indeed be seen in the lower limbs of fetuses, just like in adults. The lower limbs are not exempt from the process.
**Option D:** This option is incorrect because while it is true that rigor mortis may be less pronounced in fetuses due to the delayed onset, it is not true that it is "established but too faint to be appreciated." Rigor mortis in fetuses is typically not seen before 7 months, as mentioned earlier.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the delayed onset of rigor mortis in fetuses can be used as a clue in forensic investigations to estimate the post-mortem interval. However, it is crucial to consider other factors, such as the gestational age and the presence of any medical conditions that may affect the process.

**✓ Correct Answer: A. Usually not seen before 7 months**