You are answering a USMLE-style medical question. Question: A 15-year-old boy is brought to the physician by his mother. His mother reports multiple instances over the past month when she walked into his room while he was masturbating. She is worried he may be going through some sort of “problem.” He is currently in 9th grade; his mother says that he is the captain of his high school wrestling team and does reasonably well in school. On examination, he seems embarrassed and avoids making eye contact. Physical examination shows no abnormalities. Which of the following is the most appropriate next step in management? Options: A) Advise the patient to stop masturbating B) Measure serum testosterone level C) Reassure the mother D) Prescribe sertraline Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.