Café-au-lait Spots — Neurofibromatosis Type 1

A 6-year-old boy presents to pediatrics clinic with multiple brown macules on his trunk and axillae. On examination, the structure marked **B** in the diagram (axillary freckling) is noted. His mother reports similar lesions and has a history of multiple skin tumors. What is the most likely diagnosis, and which gene mutation is responsible for this condition?