Trisomy 8 Mosaicism

A 3-year-old boy is referred to pediatrics for developmental delay and distinctive facial features. On examination, he has a prominent forehead, deep-set eyes, broad upturned nose, and thick everted lower lip. Notably, his palms and soles show characteristic deep longitudinal furrows. He also has bilateral patella hypoplasia, camptodactyly, and restricted joint mobility. Karyotype analysis reveals 47,XY,+8/46,XY in skin fibroblasts but not in peripheral blood lymphocytes. The structure marked **A** in the diagram represents this condition. Which of the following is the most critical long-term surveillance parameter for this patient?