A 19-year-old male presents with a 1-week history of malaise and anorexia followed by fever and sore throat. On physical examination, the throat is inflamed without exudate. There are a few palatal petechiae. Cervical adenopathy is present. The liver is percussed at 12 cm and the spleen is palpable. The throat culture is negative for group A streptococci and peripheral blood smears show atypical lymphocytosis. Hb: 12 g/dL, Reticulocytes: 4% WBC: 14,000/uL with 60% Lymphocytes Bilirubin total: 2.0 mg/dL (normal 0.2 to 1.2) Lactic dehydrogenase (LDH) serum: 260 IU/L (normal 20 to 220) Aspaate (AST): 40 U/L (normal 8 to 20 U/L) Alanine (ALT): 35 U/L (normal 8 to 20 U/L) The next best investigation modality would be: