This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Penicillin & Cephalosporin, Subject name: Pharmacology
All have beta lactam ring EXCEPT: 
Select and explain. 
A. Penicillin, B. Linezolid, C. Cefotaxime, D. Imipenem
B: Linezolid
Answer: B: Linezolid 
Explanation: ANSWER: (B) LinezolidREF: Goodman and Gillman 11th edition page 780"Linezolid is a synthetic antimicrobial agent of the oxazolidinone class"The three classes of drugs which have beta lactam ring in its structure are:Penicillins consist of a thiazolidine ring connected to a b-lactam ring, to which is attached a side chainCephalosporins are produced from 7-aminocephalosporanic acid by the addition of different side chains. Modifications at position 7 of the b-Iactam ring alter antibacterial activity, while substitutions at position 3 of the dihydrothiazine ring alter the metabolism and pharmacokinetic properties.IILCarbapenems are b-lactams that have a broader spectrum of activity than most other fa- lactam antibiotics