Metabolism USMLE Step 1 Practice Question
A 4-month-old male presents with recurrent oral thrush, chronic diarrhea, and recurrent severe respiratory infections. Vital signs show HR 128/min, RR 32/min, temperature 37.2°C, SpO2 94% on room air. Laboratory studies reveal profound lymphopenia (absolute lymphocyte count 800/μL) affecting both T and B cells, with normal immunoglobulin levels. Chest X-ray shows interstitial pneumonia without thymic shadow. He has received no vaccinations. Which enzyme deficiency most likely explains this immunodeficiency?
Answer choices
- ADihydropteridine reductase
- BHypoxanthine guanine phosphoribosyltransferase
- COrnithine transcarbamylase
- DAdenosine deaminaseCorrect answer
- EGalactose 1 phosphate uridyltransferase
- FPurine nucleoside phosphorylase
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