Final answer:
Selective IgA deficiency is the most common B-cell defect that presents with recurrent URIs and diarrhea. It leads to an increased susceptibility to respiratory and gastrointestinal infections caused by various pathogens. The complications of this condition can be severe and life-threatening.
Step-by-step explanation:
The most common B-cell defect that presents with recurrent upper respiratory infections (URIs) and diarrhea is called selective IgA deficiency.
This condition affects about one in 800 people and is characterized by the inability to produce secretory IgA, which is an important defense mechanism in the respiratory and gastrointestinal tracts. As a result, individuals with selective IgA deficiency are more susceptible to infections caused by a variety of pathogens such as H. influenzae, S. pneumoniae, Moraxella catarrhalis, S. aureus, Giardia lamblia, and pathogenic strains of Escherichia coli.
The complications of selective IgA deficiency include recurrent lung and gastrointestinal infections, which can be severe and life-threatening.