Which statement accurately contrasts serosa and adventitia in GI tract walls?

• A. Serosa lines the lumen of the GI tract.
• B. Adventitia is a serous membrane.
• C. Serosa covers retroperitoneal segments.
• D. Adventitia lacks a serosa.

Answer: (D)

The contrast being tested is where a serous membrane (serosa) versus a connective tissue layer (adventitia) sits on GI tract walls and what each one does. Serosa is a serous membrane that forms the visceral peritoneum, covering intraperitoneal organs and producing lubricating fluid to reduce friction as the organs move. Adventitia, on the other hand, is a dense connective tissue layer that anchors organs to surrounding tissues and blends with nearby structures; it does not contain a serous membrane or secrete serous fluid.

In the GI tract, organs that lie within the peritoneal cavity are covered by serosa, while those that lie outside this cavity (retroperitoneal) are wrapped by adventitia instead of serosa. Because of this, adventitia lacks a serosa. The lumen itself is lined by mucosa, not by serosa, so serosa does not line the inside of the tube.

So the statement that adventitia lacks a serosa is the correct contrast.