What are the four layers of the GI tract?

• A. Mucosa, submucosa, mucosa, serosa
• B. Mucosa, submucosa, muscularis, serosa
• C. Mucosa, submucosa, muscle, serosa
• D. Mucosa, submucosa, muscle, adventitia

Answer: (C)

From the lumen outward, the GI tract has four layers: mucosa, submucosa, muscularis externa (the muscle layer), and serosa. The mucosa lines the tract and includes epithelium, lamina propria, and the muscularis mucosae. The submucosa is a connective tissue layer with blood vessels, nerves, and sometimes glands. The muscularis externa provides the contractile power for movement, with an inner circular and an outer longitudinal layer that drive peristalsis and mixing. The serosa is the outer serous membrane covering intraperitoneal portions; in retroperitoneal portions, the outer layer is adventitia instead. The option listing muscle corresponds to the muscularis externa, which is the intended fourth layer in this sequence. The other options either repeat the mucosa, swap in a different term for the muscle layer, or use adventitia in place of serosa, which doesn’t fit the standard four-layer structure.