Immunology USMLE Step 1 Practice Question
A 4-month-old breastfed infant with recurrent sick contacts at home presents with minimal infections. Vital signs are normal (HR 140, RR 35, Temp 37.2°C). Physical examination reveals no signs of active infection; stool culture is negative. The infant has not yet received rotavirus vaccination. Which immune factor in breast milk most directly protects the infant's gastrointestinal mucosa from pathogenic invasion?
Answer choices
- AMaternal complement proteins generating membrane attack complex in the lumen
- BMaternal CD8 positive T cells crossing the intestinal wall
- CMaternal IgE activating neonatal mast cells
- DMaternal IgA that resists proteolysis at mucosal surfacesCorrect answer
- EMaternal IgM absorbed intact into the bloodstream
- FMaternal IgG binding to intestinal epithelial cell Fc receptors to block pathogen adherence
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