**Core Concept**
A spigelian hernia is a rare type of abdominal hernia that occurs through a defect in the spigelian fascia, which is the aponeurotic layer between the rectus abdominis muscle and the semilunar line. This type of hernia is characterized by its unique location and pathophysiology.

**Why the Correct Answer is Right**
Spigelian hernias occur due to a defect in the transversus abdominis muscle, which is one of the three flat muscles of the abdominal wall. This defect leads to a protrusion of abdominal contents through the spigelian fascia, which lies between the rectus abdominis muscle and the semilunar line. The spigelian fascia is a weak point in the abdominal wall, making it prone to herniation. The unique location of spigelian hernias, between the muscle layers, distinguishes them from other types of hernias.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because spigelian hernias are typically located in the lower abdominal region, not in the femoral triangle region.
**Option B:** Incorrect because spigelian hernias can occur in both males and females, although they are more common in females.
**Option D:** Incorrect because spigelian hernias do not always contain the appendix, and other abdominal contents such as intestine or omentum may protrude through the defect.

**Clinical Pearl / High-Yield Fact**
Spigelian hernias are often mistaken for other types of abdominal hernias, such as incisional or umbilical hernias. However, their unique location and pathophysiology make them distinct and require prompt surgical attention to prevent complications.

**✓ Correct Answer: C. It is due to defect in transversus abdominalis muscle and lies in between muscle layers**