Before administering cefazolin (Ancef), the nurse checks for a previous allergic reaction to:

The correct answer is B: Penicillins. Cefazolin is a first-generation cephalosporin antibiotic, which shares a structural similarity to penicillins. Patients allergic to penicillins have an increased risk of cross-reactivity with cephalosporins like cefazolin due to this structural resemblance. Checking for previous allergic reactions to penicillins before administering cefazolin is crucial to prevent potential allergic reactions. Choices A, C, and D are incorrect as they do not share a close structural relationship with cefazolin, thus checking for allergies to these substances is not essential before administering cefazolin.