Antibiotic Mechanisms of Action: The USMLE Step 1 Guide

Answer: A. The t(9;22)(q34;q11.2) translocation juxtaposes the BCR gene on chromosome 22 with ABL1 on chromosome 9, producing the BCR-ABL fusion gene. This fusion protein has constitutive, ligand-independent tyrosine kinase activity that drives leukemogenic proliferation via downstream pathways (RAS/MAPK, PI3K/AKT, JAK/STAT). Imatinib is a reversible, competitive small-molecule inhibitor that occupies the ATP-binding pocket of the kinase domain, locking it in an inactive conformation and blocking phosphorylation of downstream substrates. Option B incorrectly denies the kinase's enzymatic activity, which is central to disease pathogenesis. Option C mischaracterizes imatinib as an irreversible covalent degrader rather than a reversible competitive inhibitor. Option D is incorrect because BCR-ABL is expressed and active across the myeloid lineage, which is precisely why CML originates from a pluripotent hematopoietic stem cell and involves granulocytic proliferation. Option E incorrectly attributes this translocation to APL, which is instead defined by t(15;17) PML-RARA.

Answer: B. An order that conflicts with documented allergy information must be verified with the prescriber, which addresses the potential error at its source before any harm occurs (option B). Administering the drug because the prescriber wrote it (A) assumes the allergy was seen and transfers the risk to the client. Holding the dose with no follow-up (C) leaves the concern unresolved and can delay needed therapy. Asking the client to decide (D) shifts a professional safety judgment onto someone without the clinical information. Escalation is appropriate only if clarification fails or the risk persists.

Answer: D. Hives with lip swelling and breathing difficulty indicate anaphylaxis, a life-threatening emergency, so the nurse stops the infusion at once and summons emergency help while preparing for epinephrine (option D). Slowing the infusion (A) continues to deliver the causative drug during an evolving airway emergency. Giving an oral antihistamine and waiting 15 minutes (B) is far too slow and is not the first-line treatment. Documenting and waiting for rounds (C) delays care that is needed within minutes. The nurse maintains the airway, keeps the intravenous access, and stays with the client.