The muscularis mucosae is a thin layer of smooth muscle immediately deep (external) to the:

• A. lamina propria
• B. submucosa
• C. serosa
• D. muscularis externa

Answer: (A)

Verified step by step guidance

1. Step 1: Begin by understanding the structure of the gastrointestinal tract wall, which consists of four main layers: mucosa, submucosa, muscularis externa, and serosa. Each layer has distinct components and functions.
2. Step 2: Focus on the mucosa layer, which is the innermost layer. It is composed of three sublayers: the epithelium, the lamina propria (a connective tissue layer), and the muscularis mucosae (a thin layer of smooth muscle).
3. Step 3: Note that the muscularis mucosae is located immediately deep (external) to the lamina propria within the mucosa layer. This means it lies just beneath the lamina propria and is part of the mucosa itself.
4. Step 4: Understand the other options provided in the question: the submucosa is a separate layer located external to the mucosa, the serosa is the outermost layer of the gastrointestinal tract, and the muscularis externa is a thick layer of smooth muscle responsible for peristalsis, located external to the submucosa.
5. Step 5: Based on the anatomical organization, recognize that the muscularis mucosae is immediately deep to the lamina propria, making 'lamina propria' the correct answer.