A 9- nonth-0ld presented With recurrent attacks of diarrhea, thrush ,and failure to thrive shortly after birth. You suspect immune deficiency. of the following, the MOST common cause of this condition is

(C). Children with defects in antibody production, phagocytic cells, or complement proteins have recurrent infections with encapsulated bacteria and may grow and develop normally despite their recurring infections, unlessthey develop bronchiectasis from repeated lower respiratory tract bacterial infections or persistent enteroviral infections of the central nervous system. By contrast, patients with deficiencies in T-cell function usually develop opportunistic infections or serious illnesses from common viral agents early in life, and they fail to thrive.