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: Microbiology
Mononuclear phagocytes are produced by ? 
Select and explain. 
A. Thymus, B. Spleen, C. Bone marrow, D. Liver
C: Bone marrow
Answer: C: Bone marrow 
Explanation: Ans. is 'c' i.e., Bone marrow Mononuclear macrophage cells originate in the bone marrow from precursor cells and become monocytes in about 6 days.