**Core Concept**
Retrocecal appendicitis involves inflammation of the appendix located behind the cecum, which can irritate nearby structures, including muscles. The correct answer requires knowledge of the anatomical relationship between the appendix and nearby muscles in the abdominal wall.

**Why the Correct Answer is Right**
The iliopectineal muscle, also known as the iliopsoas muscle, is responsible for hip flexion and is located in the pelvic region. Retrocecal appendicitis can irritate this muscle due to its proximity to the inflamed appendix. The iliopectineal muscle is innervated by the femoral nerve and receives blood supply from the iliolumbar artery. The inflammation from the retrocecal appendicitis can spread to the surrounding tissues, including the iliopectineal muscle, leading to pain and discomfort.

**Why Each Wrong Option is Incorrect**
**Option A:** The external oblique muscle is a superficial abdominal muscle that is not directly related to the retrocecal appendix. It is not affected by retrocecal appendicitis.
**Option B:** The internal oblique muscle is another abdominal muscle that is not directly related to the retrocecal appendix. It is not affected by retrocecal appendicitis.
**Option C:** The psoas major muscle is a deep abdominal muscle that is not directly related to the retrocecal appendix. However, it is a nearby structure that can be affected by inflammation, but it is not the most commonly irritated muscle by retrocecal appendicitis.

**Clinical Pearl / High-Yield Fact**
Retrocecal appendicitis can mimic other conditions such as renal colic, ovarian torsion, and lower gastrointestinal tract disorders due to its variable presentation. A high index of suspicion and a thorough clinical examination are essential for accurate diagnosis.

**Correct Answer:** C.