First Aid USMLE Step 1: Pharmacology (Complete) Questions with Correct Answers.

Penicillin G (IV and IM) Penicillin M (oral) Correct Answer *MOA:* *β-lactam*: a structural analog of D-ala-D-ala (a normal component of cell walls) which binds and inactivates bacterial transpeptidases (penicillin-binding protein, PBPs) to block peptidoglycan cross linking *Clinical use:* Bactericidal for Gram-⊕ cocci/rods, Gram-⊖ cocci (Neisseria), and spirochetes Mostly used for Gram-⊕ organisms (S. pneumoniae, S. pyogenes, Actinomyces). Also used for Gram-⊖ cocci (mainly N. meningitidis) and spirochetes (namely T. pallidum). Bactericidal for Gram-⊕ cocci, Gram-⊕ rods, Gram-⊖ cocci, and spirochetes. Penicillinase sensitive. *Toxicity:* Hypersensitivity reactions (type II), hemolytic anemia *MOR:* Penicilinase (a type of β-lactamase) in bacteria cleaves Oxacillin Nafcillin Dicloxacillin Correct Answer Penicillinase-resistant penicillins *MOA:* Same as penicillin. *Narrow spectrum*; penicillinase resistant because bulky R group blocks access of β-lactamase to β-lactam ring. *Clinical use:* Bactericidal, narrow spectrum: *S. aureus* (except MRSA; resistant due to altered penicillin-binding protein target site). *Toxicity:* Hypersensitivity reactions (type II), interstitial nephritis "Use *naf* (*naf*cillin) for staph." Nafcillin has penetration to CNS Aminopenicillins: Ampicillin Amoxicillin Correct Answer Penicillinase-sensitive penicillins