Infectious mononucleosis is most commonly caused by which virus?

• A. Epstein-Barr virus
• B. Cytomegalovirus
• C. Adenovirus
• D. Rhinovirus

Answer: (A)

Epstein-Barr virus is the most common cause of infectious mononucleosis. EBV is a herpesvirus that primarily infects B lymphocytes via the CD21 receptor and then establishes latency. Transmission occurs mainly through saliva, which is why the illness is often called the “kissing disease.” The classic presentation in teens and young adults includes fever, sore throat with exudative tonsillitis, lymphadenopathy (especially posterior cervical), fatigue, and sometimes hepatosplenomegaly. Blood tests often show lymphocytosis with atypical lymphocytes, and liver enzymes may be elevated; a heterophile antibody test (Monospot) is typically positive, though it can be negative early in infection or in young children. While cytomegalovirus can cause a mono-like syndrome, and adenovirus or rhinovirus can produce pharyngitis, EBV remains the most typical and characteristic cause of infectious mononucleosis due to its pattern of pharyngitis, lymphadenopathy, and fatigue.