Medical examination of a 19-year-old worker revealed generalized lymphadenopathy mainly affecting the posterior cervical, axillary and ulnar lymph nodes. There are multiple injection marks on the elbow bend skin. The man denies taking drugs, the presence of injection marks ascribes to influenza treatment. Blood count: RBCs- 3,2 × 1012/l, Hb- 100 g/l, WBCs- 3,1 × 109/l, moderate lymphopenia. What study is required in the first place?