In a GI tract wall image, which layer is missing?

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

Answer: (A)

Understanding the layered structure of the GI tract wall helps explain why a layer might be missing in a histology image. The wall is organized from the inside out as mucosa (epithelium with lamina propria and often muscularis mucosae), submucosa, muscularis externa (inner circular and outer longitudinal smooth muscle), and serosa (or adventitia in some regions).

If the image shows mucosa and submucosa but lacks the thick, two-layer smooth muscle coat, the missing layer is the muscularis externa. This often happens with shallow biopsies or sections prepared in a way that doesn’t include the deeper muscular layers, leaving the mucosa and submucosa visible while the muscularis externa isn’t present.

The other layers would not typically be absent in this context: the mucosa would appear as the inner lining, and the submucosa would lie beneath it; serosa would be the outermost covering in regions where present, but its absence is less likely in standard mucosal biopsies.