The most common primary immunodeficiency is:

Ans. is 'b' i.e., Isolated IgA immunodeficiency(Ref: Harrison, 18th/e, p. 2704 and 17th/e, p. 2058)Selective IgA deficiency is the most common primary immunodeficiency, familial (more common), naive B cells are not able to differentiate into IgA producing cells. Along with IgA, IgG2 and IgG4 may also be reduced.