A urinary tract infection with an extended-spectrum beta-lactamase (ESBL)-producing E. coli fails to respond to cephalosporins. The ESBL enzyme responsible most commonly belongs to which structural enzyme class?
- A Ambler class A serine beta-lactamases (SHV, TEM, CTX-M family) ✓
- B Ambler class B metallo-beta-lactamases requiring zinc cofactor
- C Ambler class C AmpC cephalosporinases that are chromosomally encoded
- D Ambler class D oxacillinases that preferentially hydrolyze carbapenems
Explanation
ESBLs in clinical Enterobacteriaceae are predominantly Ambler class A serine enzymes derived from TEM and SHV parents with mutations expanding their spectrum, or the globally dominant CTX-M family which hydrolyzes third-generation cephalosporins particularly well. Ambler class B metallo-beta-lactamases (NDM, VIM, IMP) hydrolyze carbapenems and are carbapenemases, not ESBLs. AmpC cephalosporinases (class C) are another distinct resistance mechanism. Class D (OXA-type) carbapenemases are important in Acinetobacter. The distinction matters because carbapenems are the treatment of choice for ESBL infections, whereas metallo-BL producers require ceftazidime-avibactam-aztreonam combinations.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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