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
Metanephros develops during which week of embryonic development? 
Select and explain. 
A. 4th, B. 5th, C. 6th, D. None of the above
B: 5th
Answer: B: 5th 
Explanation: The third urinary organ, the metanephros, or permanent kidney, appears in the fifth week. Its excretory units develop from metanephric mesoderm in the same manner as in the mesonephric system. Ref: Langman's embryology 11th edition Chapter 15.