This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
All drug inhibit bacterial cell wall synthesis EXCEPT 
Select and explain. 
A. Spectinomycin, B. Vancomycin, C. Aztreonam, D. Cephalexin
A: Spectinomycin
Answer: A: Spectinomycin 
Explanation: Spectinomycin is an aminocyclitol antibiotic that inhibits bacterial protein synthesis. The other drugs all inhibit bacterial cell wall synthesis. Vancomycin and bacitracin inhibit early steps in the biosynthesis of the peptidoglycan component of the cell wall, whereas 3-- lactams such as aztreonam (a monobactam), penicillins, cephalosporins, and carbapenems inhibit the cross-linking (transpeptidation) of the cell wall peptidoglycan polymers.