Immunology USMLE Step 1 Practice Question
A 4-year-old girl presents with recurrent otitis media, pneumonia, and two episodes of septicemia since infancy. Vital signs show BP 95/60, HR 112, RR 24, Temp 38.2°C, SpO2 98%. She develops hematuria and proteinuria following streptococcal pharyngitis. Serum creatinine is 0.8 mg/dL. She denies rashes or joint pain. Complete blood count and immunoglobulin levels are normal. Which complement abnormality most likely explains these recurrent infections and post-infectious glomerulonephritis?
Answer choices
- ADeficiency of decay accelerating factor on red blood cells
- BDeficiency of C5 through C9 causing impaired membrane attack complex formation
- CDeficiency of C1 esterase inhibitor causing increased bradykinin
- DDeficiency of factor XII
- EDeficiency of C3 causing impaired opsonizationCorrect answer
- FDeficiency of complement factor I causing unopposed C3b degradation
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