Name the layers of the GI tract from lumen outward in order.

• A. Mucosa
• B. Submucosa
• C. Muscularis externa
• D. Adventitia/Serosa

Answer: (D)

From the inside to the outside, the GI tract wall is built in four main layers. The innermost lining, the mucosa, comes into direct contact with the lumen contents and includes epithelium, a supportive lamina propria, and a thin muscular layer called the muscularis mucosae. Next is the submucosa, a dense connective tissue layer that carries blood vessels, nerves, and sometimes glands. Outside that lies the muscularis externa, responsible for movement and propulsion; it typically has an inner circular layer and an outer longitudinal layer, coordinated by the enteric nervous system. The outermost covering is the serosa, a smooth membrane that reduces friction for parts of the tract inside the peritoneal cavity, while areas outside the peritoneal cavity have Adventitia, a connective tissue layer that blends with surrounding structures.