This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Haematology, Subject name: Pathology
A 23-year-old male presents with fever and cervical lymphadenopathy. The peripheral smear shows the following finding. Diagnosis is 
Select and explain. 
A. AML, B. ALL, C. Infectious mononucleosis, D. None of the above
C: Infectious mononucleosis
Answer: C: Infectious mononucleosis 
Explanation: Infectious mononucleosis *Causative agent: Ebstein Barr virus (HHV-4)*Mode of transmission: Direct contact with human secretions, most often by saliva *Infected cells: Epithelial cells and B cells in the oropharynx*EBV infects B cells through C3d complement receptor (CD21) for which the co-factor used is MHC-II.*By age of 3 years, about 100% of children have positive serological findings for EBV. But not all those infected individuals acquire the disease.*EBV replicates in B cells, inducing expression of viral capsid antigen (VCA), membrane antigen (MA), early antigen (EA), EBV nuclear antigens (EBNA), and lymphocyte-detected membrane antigen (LYDMA).*Antibodies formed against these antigens were called heterophil antibodies which play a role in containing the infection*Incubation period: 2 to 5 weeks *Triad of symptoms: Fever, pharyngitis, and cervical /generalized lymphadenopathy*Peripheral blood: Shows atypical lymphocytes which are T cell type (however EBV affects B cells) with abundant pale blue cytoplasm, large nuclei and conspicuous nucleoli called Downey cells *Paul bunnel test: Agglutination of sheep erythrocytes by patient's serum. Beef erythrocytes (specific for infectious mononucleosis ) and guinea pig kidney are also used. *Monospot test: Most sensitive test in which agglutination was formed between horse erythrocytes and patient's serum.*Serology: To detect EBV specific antibodies VCA (IgM), VCA (IgG), EA and EBNA.Ref: Iochim's lymph node pathology p78