A transplant patient who had serologic evidence of previous Epstein-Barr virus infection was taking high levels of immunosuppressive medications. He presents with generalized lymphadenopathy, fever, night sweats, weight loss, abdominal pain, and tonsillitis. The dosage of immunosuppressive drugs given to the patient is decreased, and the lymphadenopathy regresses. Which of the following is the best diagnosis for this patient?