**Core Concept:**
The ischiorectal fossa is a potential space located between the ischium and the rectum, which helps in the spread of pathogens during infections. The lateral border of the ischiorectal fossa is formed by the iliacus muscle, which is part of the external oblique muscle group in the anterior abdominal wall.

**Why the Correct Answer is Right:**
The lateral border of the ischiorectal fossa is formed by the iliacus muscle, which originates from the anterior superior iliac spine and inserts onto the lesser trochanter of the femur. This muscle plays a crucial role in the spread of pathogens during infections, as it helps to spread the infection from the ischiorectal fossa to the adjacent structures such as the inguinal canal and pelvic sidewalls.

**Why Each Wrong Option is Incorrect:**
A. The iliopsoas muscle (iliacus and psoas muscles) is the correct answer, not the iliacus muscle alone.
B. The piriformis muscle is not involved in the formation of the lateral border of the ischiorectal fossa.
C. The levator ani muscle is part of the pelvic floor muscles and does not contribute to the formation of the lateral border of the ischiorectal fossa.
D. The gluteus maximus muscle is a large muscle in the gluteal region and is not involved in the formation of the lateral border of the ischiorectal fossa.

**Clinical Pearl:**
Understanding the anatomy of the ischiorectal fossa and its neighboring muscles is essential for diagnosing and treating infections in the pelvic region. A thorough knowledge of the anatomy helps in recognizing the spread of infection and selecting appropriate surgical approaches to manage pelvic abscesses and pelvic inflammatory disorders.