Café-au-lait Macules + Axillary Freckling — NF1

A 5-year-old boy is brought to the pediatric clinic by his mother for evaluation of multiple brown patches on his skin. On examination, the child has 7 well-demarcated, uniformly tan macules distributed over his trunk and limbs, each measuring 8–12 mm in greatest diameter. The structure marked **A** in the diagram represents these lesions. Which of the following is the MOST important next step in establishing a diagnosis of Neurofibromatosis Type 1 (NF1) in this child?