Human T-lymphotropic virus

A 47-year-old woman comes to her physician for assessment of painless “swellings” over her body for the past several months. Additionally, she endorses abdominal pain, constipation, fatigue, and cognitive slowing for several weeks. She recently immigrated from Japan last year, and this is her first visit with a physician in the United States. She has a remote history of intravenous drug use but does not currently use. Vitals are within normal limits. Physical examination reveals generalized lymphadenopathy. Splenomegaly is present. CT scan of the chest, abdomen, and pelvis reveals generalized diffuse lymph node enlargement and lytic bone lesions. Laboratory findings demonstrated below:  


HIV testing is negative. A peripheral blood smear shows lymphocytes with condensed chromatin and hyperlobulated nuclei. Which of the following is the most likely etiology of this patient’s condition?