added nbme12 tag b/c directly answers the Q:
A 13-month-old boy is brought to the physician for a follow-up examination. He completed a 10-day course of amoxicillin for an episode of pneumonia 10 days ago. He has had four episodes of pneumonia since birth and had severe chickenpox at the age of 8 months. He is below the 5th percentile for length and weight. His temperature is 37.4°C (99.3°F), pulse is 120/min, respirations are 30/min, and blood pressure is 94/50 mm Hg. Examination shows no abnormalities except for oral thrush. Laboratory studies show a leukocyte count of 1500/mm (50% segmented neutrophils, 8% eosinophils, 1% basophils, 35% lymphocytes, and 6% monocytes), a CD4+ T-lymphocyte count of 225/mm (Normal≥500), and a low lymphocyte proliferative response. Which of the following is the most likely diagnosis?
Ans: Combined immunodeficiency
Combined immunodeficiency describes a syndrome of failed B- and T-lymphocyte activity leading to impaired cellular and humoral immunity.