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
Which among the following drug is safest in a patient allergic to penicillin: 
Select and explain. 
A. Cephalexin, B. Imipennem, C. Cefepime, D. Aztreonam
D: Aztreonam
Answer: D: Aztreonam 
Explanation: Aztreonam