This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Abdomen and pelvis, Subject name: Anatomy
Vaginal epithelium is a derivative of 
Select and explain. 
A. Mesoderm of urogenital sinus, B. Endoderm of urogenital sinus, C. Mesoderm of genital ridge, D. Endoderm of genital ridge
B: Endoderm of urogenital sinus
Answer: B: Endoderm of urogenital sinus 
Explanation: The wall of the vagina from the lumen outwards consists firstly of the mucosa of non keratinized stratified squamous epithelium with an underlying lamina propria of connective tissue, secondly a layer of smooth muscle with bundles of circular fibers internal to longitudinal fibers, and thirdly an outer layer of connective tissue called the adventitia. ref - BDC 6e vol2 pg395