What measurement should be taken to determine if a child has microcephaly during a routine exam?

To determine if a child has microcephaly during a routine exam, measuring head circumference is essential. Microcephaly is a condition characterized by an abnormally small head size, which can be indicative of underlying developmental issues. The head circumference measurement provides direct insight into the growth and development of the child's brain and skull relative to expected norms for their age and sex.

Height and weight measurements are critical for assessing a child's overall growth and nutritional status, but they do not directly evaluate brain size. Similarly, arm span measurements are not relevant when diagnosing microcephaly, as they pertain more to the assessment of skeletal growth rather than head development. Hence, head circumference is the specific measurement used to identify cases of microcephaly.