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Croup — USMLE Step 3 MCQ

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HardPediatrics (well-child, chronic management)CroupUSMLE Step 3

A 32-year-old with known G6PD deficiency develops cyanosis, confusion, chocolate-brown blood, and methemoglobin 38% after topical benzocaine. High-flow oxygen is started, but neurologic status worsens. Which next management is most appropriate?

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Correct answer: B — Use exchange transfusion or hyperbaric oxygen with toxicology consultation

The best answer is “Use exchange transfusion or hyperbaric oxygen with toxicology consultation”. Methylene blue depends on NADPH and can be ineffective or provoke hemolysis in G6PD deficiency. This patient has severe symptomatic methemoglobinemia despite oxygen, so toxicology-guided exchange transfusion or hyperbaric oxygen is appropriate. Hydroxocobalamin treats cyanide poisoning, and the clinical severity—not an arbitrary 50% threshold—requires escalation. The alternatives are less suitable because they mistime the intervention, omit a necessary component, or apply treatment for a different physiologic problem; the stem's decisive findings determine the sequence.

Reference: CDC/ATSDR, Aniline Medical Management Guideline: https://wwwn.cdc.gov/tsp/MMG/MMGDetails.aspx?mmgid=448&toxid=79