C) Reassure the parents and schedule a routine follow-up visit

The anterior fontanelle typically closes by 18 months of age, and it can be normal for it to remain open until this age. Given that the child is otherwise healthy and meeting developmental milestones, reassurance and follow-up are appropriate.

Answer choice A: Check serum calcium levels, is incorrect. While hypocalcemia can delay fontanelle closure, there is no indication of calcium imbalance in this otherwise healthy child.

Answer choice B: Obtain a head ultrasound, is incorrect. A head ultrasound is not necessary in this case as there are no signs of increased intracranial pressure or other abnormalities.

Answer choice D: Refer to a pediatric endocrinologist, is incorrect. There is no indication of endocrine disorders in this child, making this referral unnecessary.

Answer choice E: Schedule a cranial MRI, is incorrect. A cranial MRI is not warranted in the absence of neurological symptoms or other concerning findings.

Key Learning Point

The anterior fontanelle is usually closed by 18 months of age.