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: Medicine
A patient is found to be positive only for Anti HBsAg. All other viral markers are negative.The likely diagnosis is : 
Select and explain. 
A. Vaccination, B. Chronic hepatitis B, C. Acute hepatitis B, D. Fulminant hepatitis B
A: Vaccination
Answer: A: Vaccination 
Explanation: Answer is A (Vaccination) Isolated Anti-HBs Ag in serum is seen in immunization with HB.s Ag. Possible Causes of Isolated Anti-HBsAg Positivity in serum (Other markers negative) Immunization with HBsAg (after vaccination) Hepatitis B in the remote past (?) False-positive