This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastro Intestinal System, Subject name: Pediatrics
A 5-year-old boy is detected to be HBsAg positive on two separate occasions during a screening program for hepatitis B. He is otherwise asymptomatiC. Child was given three doses of recombinant hepatitis B vaccine at the age of 1 year. His mother was treated for chronic hepatitis B infection around the same time. The next relevant step for further investigating the child would be to: 
Select and explain. 
A. Obtain HbeAg and anti-Hbe levels, B. Obtain anti-Hbs levels, C. Repeat HbsAg, D. Repeat another course of Hepatitis B vaccine
B: Obtain anti-Hbs levels
Answer: B: Obtain anti-Hbs levels 
Explanation: B. Obtain anti Hbs levels(Ref: Nelson's 20/e p 1942-1952, Ghai 8/e p 220-224)The child in question has received complete vaccination for Hepatitis B, is currently asymptomatic but has tested positive for HBsAg. In order to know whether the infection is acute or chronic, as well as for knowing the immune status of child, anti HBsAg levels should be done next.