## **Core Concept**
The question tests knowledge of pelvic anatomy, specifically the structures passing through the greater and lesser sciatic foramina. These foramina are significant as they allow passage of various nerves and vessels between the pelvic cavity and the gluteal region or perineum.

## **Why the Correct Answer is Right**
The correct answer, **Piriformis muscle**, is a key structure that passes through the greater sciatic foramen. However, when considering a structure that passes through **both** the greater and lesser sciatic foramina, we focus on the **Pudendal nerve**. The pudendal nerve originates from the sacral nerve roots (S2-S4), exits the pelvic cavity through the greater sciatic foramen (below the piriformis muscle), and then re-enters the pelvic cavity through the lesser sciatic foramen. This unique path allows it to innervate the external genitalia and provide motor innervation to the muscles of the perineum.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While the **Sciatic nerve** does pass through the greater sciatic foramen, it does not pass through the lesser sciatic foramen.
- **Option B:** The **Superior gluteal nerve** exits through the greater sciatic foramen above the piriformis muscle but does not pass through the lesser sciatic foramen.
- **Option C:** The **Inferior gluteal nerve** passes through the greater sciatic foramen below the piriformis muscle but does not pass through the lesser sciatic foramen.

## **Clinical Pearl / High-Yield Fact**
A useful clinical correlation to remember is the **Pudendal nerve block**, which is performed by injecting anesthetic near the ischial spine (or through the vaginal wall for pudendal nerve block in obstetrics) to anesthetize the pudendal nerve. This procedure is useful for pain relief during childbirth or for minor gynecological and urological procedures.

## **Correct Answer Line**
**Correct Answer: D. Pudendal nerve**