This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Integrated QBank, Subject name: Unknown
A 3-year-old child presented with fatigue, malaise, fever, sore throat, headache, nausea, abdominal pain and myalgia. O/E Generalized lymphadenopathy Hepatosplenomegaly, Tonsillar enlargement Palatal petechiae Rashes and edema of the eyelids. Some weird cells were observed in the peripheral blood. A specific serological test was performed Which of the following receptor is involved in the pathogenesis of the above disease: - 
Select and explain. 
A. CD21, B. CD20, C. CD19, D. CD117
A: CD21
Answer: A: CD21 
Explanation: This is a case of Infectious mononucleosis. So, there are no B-cells in X-linked agammaglobulinemia to be affected. 1st image shows tonsillitis with membrane formation Peripheral blood film shows atypical lymphocytes or Downey cells are lymphocytes that become large as a result of antigen stimulation. B cells are the main reservoir of infection. An EBV envelope glycoprotein binds CD21 (CR2), the receptor for the C3d component of complement, which is present on B cells.