This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: New born infants, Subject name: Pediatrics
Microcephaly is defined as head circumference _________ 
Select and explain. 
A. < 1SD for age and sex, B. < 2SD for age and sex, C. < 3SD for age and sex, D. < 4SD for age and sex
C: < 3SD for age and sex
Answer: C: < 3SD for age and sex 
Explanation: *Microcephaly is defined as an occipitofrontal circumference more than 3 standard detions (SD) below the mean for given age, sex and gestation. *Causes are structural brain malformations, inherited syndromes, congenital or acquired infections, hypoxic ischemic insults and rarely metabolic disorders. Isolated inherited microcephaly is autosomal recessive. Reference : page 36 Ghai Essential Pediatrics 9 th edition