A 19-year-old girl complains of weakness, moderate headache, and sore throat when swallowing. She has been ill for 3 days already. Objectively, her body temperature is 38.2°C. Generalized lymphadenopathy, signs of tonsillopharyngitis, and moderate hepatosplenomegaly are observed. Complete blood count detected 35% of atypical mononuclear cells. What study should she be referred for, to verify the diagnosis?