Which option is NOT considered an anatomical structure?

The obturator foramen is a large opening in the pelvis formed by the ischium and pubis bones. While it is a significant feature in the anatomical landscape of the pelvis, it is classified as a space or opening rather than a structure made up of muscle, bone, or connective tissue. In contrast, the obturator externus, obturator internus, and rectus abdominis are all muscle structures. The obturator externus and internus are muscles associated with hip movement and stability, while the rectus abdominis is a prominent muscle that plays a critical role in core stability and flexion of the vertebral column. Recognizing these distinctions helps clarify why the obturator foramen does not fit with the other options as an anatomical structure.