A 29-year-old man with pe...

A 29-year-old man with perinatally acquired human immunodeficiency virus (HIV) infection and intermittent adherence to antiretroviral therapy presented to the hospital with abdominal pain and drenching night sweats. On presentation, his CD4 count was 18 cells per cubic millimeter (reference range, 500 to 1500), and the HIV viral load was undetectable. Physical exam showed severe abdominal distention, splenomegaly, and diffuse abdominal tenderness to palpation. Computed tomography (attached) of the abdomen confirmed massive splenomegaly with multifocal infarction of the splenic parenchyma. What is the most likely diagnosis/etiology?