Which muscles form the lateral margins of the true pelvis surrounding the obturator foramen at the level of the vagina and lateral to the ovaries?

• A. Obturator internus
• B. Piriformis
• C. Iliopsoas
• D. Levator ani

Answer: (A)

The lateral margins of the true pelvis around the obturator foramen are formed by the obturator internus muscle. This muscle lines the lateral pelvic wall and originates from the internal surface of the obturator membrane and surrounding bones, then its tendon courses to the greater trochanter of the femur. In the female pelvis, near the level of the vagina and beside the ovaries, the obturator internus provides the fibromuscular boundary that surrounds and completes the lateral aspect of the foramen, forming the lateral wall of the true pelvis in that region.

The other muscles do not fulfill this role: the piriformis is a posterior wall muscle, extending from the sacrum to the femur; the iliopsoas runs along the anterior/lateral aspect of the pelvis but does not define the lateral margins around the obturator foramen; and the levator ani forms the pelvic floor, not the lateral pelvic wall.