Question:hard
All of the following pass behind the ischial spine EXCEPT?
All of the following pass behind the ischial spine EXCEPT?
Show Hint
Three of these structures re-enter the perineum through the lesser sciatic foramen; one leaves the pelvis through the obturator canal instead.
Updated On: Jul 8, 2026
- Obturator nerve
- Pudendal nerve
- Internal pudendal vessels
- Nerve to obturator internus
Show Solution
The Correct Option is A
Solution and Explanation
Step 1: Group the pelvic exit routes.
Structures leaving the pelvis take one of two main doors, either the greater sciatic foramen or the obturator canal, and these two doors are on opposite sides of the pelvis.
Step 2: Follow the greater sciatic foramen group.
The pudendal nerve, the internal pudendal vessels, and the nerve to obturator internus all exit through the greater sciatic foramen, curl around the ischial spine and sacrospinous ligament, and go back in through the lesser sciatic foramen to reach the ischioanal fossa. This detour around the spine is what the question is pointing at.
Step 3: Follow the obturator nerve.
The obturator nerve takes the other door. It runs down along the side wall of the pelvis and slips out through the obturator canal, near the top of the obturator foramen, at the front of the pelvis, well away from the ischial spine.
Step 4: Spot the mismatch.
Since three of the four structures share the sciatic spine detour and the obturator nerve does not, the obturator nerve is the exception.
\[ \boxed{\text{Obturator nerve}} \]
Structures leaving the pelvis take one of two main doors, either the greater sciatic foramen or the obturator canal, and these two doors are on opposite sides of the pelvis.
Step 2: Follow the greater sciatic foramen group.
The pudendal nerve, the internal pudendal vessels, and the nerve to obturator internus all exit through the greater sciatic foramen, curl around the ischial spine and sacrospinous ligament, and go back in through the lesser sciatic foramen to reach the ischioanal fossa. This detour around the spine is what the question is pointing at.
Step 3: Follow the obturator nerve.
The obturator nerve takes the other door. It runs down along the side wall of the pelvis and slips out through the obturator canal, near the top of the obturator foramen, at the front of the pelvis, well away from the ischial spine.
Step 4: Spot the mismatch.
Since three of the four structures share the sciatic spine detour and the obturator nerve does not, the obturator nerve is the exception.
\[ \boxed{\text{Obturator nerve}} \]
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