During the second phase of midgut rotation, the small intestine rotates around the superior mesenteric artery (SMA) by how many degrees in a counterclockwise direction?

Explanation

## Midgut Rotation Phases Gut rotation occurs in three phases during weeks 6–10 of embryonic development: ### Phase 1: Initial Rotation (Week 6) - The midgut herniates out of the abdominal cavity into the umbilical cord. - The duodenojejunal flexure (DJF) rotates **90 degrees counterclockwise** around the SMA axis. - The ileocecal junction (ICJ) lies anterior to the SMA at this stage. ### Phase 2: Continued Rotation (Weeks 7–8) - During the second phase specifically, the midgut rotates an **additional 180 degrees counterclockwise** around the SMA. - This brings the cumulative total to 270 degrees counterclockwise (90° Phase 1 + 180° Phase 2). - The ICJ moves from anterior to posterior and then toward the right lower quadrant. - The duodenum becomes retroperitoneal. ### Phase 3: Final Rotation (Weeks 9–10) - The midgut returns into the abdominal cavity. - The final configuration is achieved with the DJF in the left upper quadrant (at the ligament of Treitz) and the ICJ in the right lower quadrant. **Key Point:** The stem asks specifically about the **second phase** of midgut rotation. During Phase 2, the rotation is **180 degrees counterclockwise** around the SMA. The total rotation across all phases is 270 degrees, but Phase 2 alone accounts for 180 degrees. **High-Yield:** Remember: **90° (Phase 1) + 180° (Phase 2) = 270° total** rotation around the SMA axis. When a question asks about Phase 2 specifically, the answer is **180 degrees** (option A). **Mnemonic:** **"SMA Spin"** — The midgut spins **180 degrees** in Phase 2 around the SMA, like a half-turn of a spinning top. *(Reference: Sadler TW, Langman's Medical Embryology, 14th ed.; Moore KL, The Developing Human, 10th ed.)*