This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
Anal opening is formed in which week of embryonic development: 
Select and explain. 
A. 5th, B. 7th, C. 10th, D. None of the above
B: 7th
Answer: B: 7th 
Explanation: The hindgut terminates temporarily at an ectodermal-endodermal membrane, the cloacal membrane. This membrane separates the upper pa of the anal canal, derived from endoderm, from the lower pa, called the proctoderm that is formed by an invagination pit lined by ectoderm. The membrane breaks down in the seventh week to create the opening for the anus. Ref: Langman's embryology 11th edition Chapter 7.