The most likely organ in the bullet's trajectory is the descending colon. The descending colon is a retroperitoneal structure located in the left abdomen.
In this patient with a bullet wound to the left abdomen with involvement of a retroperitoneal structure, the most likely affected organ among those listed is the descending colon. The retroperitoneum is the portion of the abdominal cavity that lies behind (retro-) the parietal peritoneum and anterior to the transversalis fascia. A useful mnemonic for remembering which structures are retroperitoneal is SADPUCKER: Suprarenal (adrenal) glands; Aorta and inferior vena cava (IVC); Duodenum (except for the most proximal aspect which is intraperitoneal); Pancreas (head, neck, and body); Ureters, Colon (ascending and descending); Kidneys; Esophagus (lower 2/3); and Rectum (upper 2/3).
Answer 2: The transverse colon is an intraperitoneal structure.
Answer 3: Although the ascending colon is a retroperitoneal structure, it is less likely to be injured in this case as it is located on the right side of the body.
Answer 4: The duodenum is divided into four parts: first (superior), second (descending), third (inferior), and fourth (ascending). All parts of the duodenum are retroperitoneal except for the most proximal part of the superior duodenum.
Answer 5: The sigmoid colon is an intraperitoneal structure.
Bullet Summary: The retroperitoneum is the anatomic space located posterior to the parietal peritoneum. It contains key organs including the adrenal glands, aorta, IVC, distal duodenum, head, neck, and body of the pancreas, ureters, ascending and descending colon, kidneys, lower esophagus, and upper rectum.