This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Common variable deficiency is due to - 
Select and explain. 
A. Absent B cells, B. Reduced number of B cells, C. Defective B cell differentiation, D. All of the above
C: Defective B cell differentiation
Answer: C: Defective B cell differentiation 
Explanation: Most patients with common variable immunodeficiency have normal or near-normal numbers of B cells in the blood and lymphoid tissues.
These B cells, however, are not able to differentiate into plasma cells.
The clinical manifestations are caused by antibody deficiency.
The feature common to all patients is hypogammaglobulinemia, generally affecting all the antibody classes but sometimes only IgG.