Which sequence below represents the correct order of the tunics of the GI tract, starting from the layer adjacent to the lumen and moving superficially?

• A. mucosa, muscularis externa, submucosa, serosa
• B. serosa, mucosa, muscularis externa, submucosa
• C. submucosa, mucosa, muscularis externa, serosa
• D. mucosa, submucosa, muscularis externa, serosa
• E. muscularis externa, serosa, submucosa, mucosa

Answer: (D)

Verified step by step guidance

1. Begin by understanding the structure of the gastrointestinal (GI) tract, which is composed of four main layers or tunics.
2. The innermost layer adjacent to the lumen is the mucosa. This layer is responsible for secretion and absorption and consists of three sub-layers: the epithelium, lamina propria, and muscularis mucosae.
3. Next is the submucosa, a layer of connective tissue that contains blood vessels, nerves, and glands. It provides support and nourishment to the mucosa.
4. The third layer is the muscularis externa, which is responsible for peristalsis and segmentation. It typically consists of two layers of smooth muscle: an inner circular layer and an outer longitudinal layer.
5. The outermost layer is the serosa, which is a protective layer. In areas of the GI tract that are not covered by serosa, the adventitia serves a similar protective function.