**Core Concept**
The rectus sheath is a fibrous envelope that encloses the rectus abdominis muscle, formed by the aponeuroses of the external and internal oblique and transverse abdominis muscles. The inferior border of the rectus sheath is of particular interest due to its anatomical variations and clinical significance.

**Why the Correct Answer is Right**
The arcuate line, also known as the arcuate line of Douglas, is an important anatomical landmark that marks the inferior border of the rectus sheath posteriorly. It is a curved line that runs transversely across the posterior wall of the rectus sheath, approximately 1-2 cm above the inguinal ligament. The arcuate line is significant because it marks the transition from the posterior rectus sheath to the aponeurosis of the transversus abdominis muscle. This is a crucial distinction, as the posterior rectus sheath is typically composed of the aponeuroses of the external and internal oblique muscles, while the aponeurosis of the transversus abdominis muscle forms the anterior layer of the posterior rectus sheath.

**Why Each Wrong Option is Incorrect**
* **Option A:** The falx inguinalis is a fold of peritoneum that hangs down from the caudal edge of the inferior epigastric artery, but it is not related to the inferior border of the rectus sheath.
* **Option B:** The inguinal ligament is a fibrous band that forms the floor of the inguinal canal, but it is not the inferior border of the rectus sheath.
* **Option C:** The internal inguinal ring is an opening in the inguinal ligament through which the spermatic cord or round ligament passes, but it is not related to the rectus sheath.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the arcuate line marks the transition from the posterior rectus sheath to the aponeurosis of the transversus abdominis muscle, which can be an important landmark for surgeons and radiologists performing abdominal procedures.

**✓ Correct Answer: D. Arcuate line**