This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urinary System: Functional Anatomy and Urine Formation, Subject name: Physiology
Each kidney contains about ___nephrons: (Repeat) 
Select and explain. 
A. One million, B. Two million, C. Four million, D. Half million
A: One million
Answer: A: One million 
Explanation: Ans: A (One million) Ref: Guyton & Hall. Physiologic Anatomy of the Kidneys. In: Textbook of Medical Physiology. 12th ed. Philadelphia: Saunders Elsevier; 2012. p 305.Explanation:Each kidney in the human contains about 800,000 to 1,000,000 nephrons, each capable of forming urineThe kidney cannot regenerate new nephrons.After age 40. the number of functioning nephrons usually decreases about 10 % every 10yearsAt age 80. many people have 40 % fewer functioning nephrons than they did at age 40.This loss is not life threatening because adaptive changes in the remaining nephrons allow them to excrete the proper amounts of water, electrolytes, and waste products