## **Core Concept**
The rectus sheath is a fibrous compartment that encloses the rectus abdominis muscle and is formed by the aponeuroses of the transverse abdominal, external oblique, and internal oblique muscles. The posterior wall of the rectus sheath is of particular interest in surgical anatomy. Below a certain level, the posterior wall's composition changes.

## **Why the Correct Answer is Right**
The correct answer, **C. Transversalis fascia**, is right because below the level of the arcuate line of Douglas (which is typically below the anterior superior iliac spine), the posterior wall of the rectus sheath is formed solely by the **transversalis fascia**. The arcuate line of Douglas marks the lower border of the posterior wall of the rectus sheath, below which the aponeuroses of the internal oblique and transverse abdominal muscles pass anterior to the rectus abdominis muscle, leaving only the transversalis fascia posterior to it.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the **external oblique aponeurosis** contributes to the anterior wall of the rectus sheath, not the posterior wall.
- **Option B:** This option is incorrect because, although the **internal oblique aponeurosis** does contribute to the rectus sheath, below the level of the arcuate line, it passes anterior to the rectus muscle, not forming the posterior wall.
- **Option D:** This option is incorrect because the **linea alba**, formed by the interlacing of the aponeuroses of the external oblique, internal oblique, and transverse abdominal muscles, runs along the midline and does not form the posterior wall of the rectus sheath.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is the location of the arcuate line of Douglas. It is an important landmark during surgical procedures in the lower abdomen, as it marks the level below which the posterior wall of the rectus sheath is significantly thinned, consisting only of the transversalis fascia. This makes it a site of potential weakness.

## **Correct Answer:** C. Transversalis fascia