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
Minimum number of mycobacteria required to be detected by AFB staining is (per ml): 
Select and explain. 
A. 15-20, B. 150-200, C. 10,000, D. I Lac
C: 10,000
Answer: C: 10,000 
Explanation: A presumptive diagnosis is commonly based on the finding of AFB on microscopic examination of a diagnostic specimen. At least 10,000 acid fast bacilli should be present per ml of sputum to be readily demonstrable in direct smears. A negative repo should not be considered till at least 300 fields have been examined, taking about 10 minutes. A positive repo can be taken only if two or more typical bacilli have been seen. Ref: Textbook of Microbiology By Ananthanarayan And Paniker, 7th edition, Page 357.