Cefalexin is the chemical structure associated with which antibiotic class?

• A. Macrolides
• B. Quinolones
• C. Cephalosporin
• D. Penicillin

Answer: (C)

Cephalexin is a cephalosporin, a member of the beta-lactam antibiotics. Its defining feature is the core chemical structure: a beta-lactam ring fused to a six-membered dihydrothiazine ring, which distinguishes cephalosporins from penicillins (which have a beta-lactam ring fused to a thiazolidine ring). This structure enables the mechanism of action of cephalosporins: they bind penicillin-binding proteins and disrupt the cross-linking of peptidoglycan in the bacterial cell wall, leading to cell lysis. Other drug classes like macrolides and quinolones act very differently and do not share this core structure, so they are not cephalosporins. First-generation cephalosporins, including cephalexin, typically cover staphylococci and streptococci well and have some activity against certain gram-negative rods. So the chemical structure in question corresponds to the cephalosporin class.