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: Pediatrics
Microcephaly is defined as 
Select and explain. 
A. Head circumference < -3SD below the mean for age and sex, B. Head circumference < -2SD below the mean for age and sex, C. Head circumference below 10th percentile the mean for age and sex, D. Head circumference below 15th percentile the mean for age and sex
A: Head circumference < -3SD below the mean for age and sex
Answer: A: Head circumference < -3SD below the mean for age and sex 
Explanation: Macrocephaly : Head circumference > 2SD above the mean for age and sex