This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cell Wall Synthesis Inhibitors, Subject name: Pharmacology
A 36 years old woman recently treated for leukemia is admitted to the hospital with malaise, chills and high fever. Gram stain of blood reveals the presence of gram negative bacilli. The initial diagnosis is bacteremia and parenteral antibiotics are indicated. The record of the patient reveals that she had severe uicarial rash, hypotension and respiratory difficulty after oral pencillin V about 6 months ago. The most appropriate drug should be: 
Select and explain. 
A. Ampicillin plus sulbactum, B. Aztreonam, C. Cefazolin, D. Imipenem plus cilastatin
B: Aztreonam
Answer: B: Aztreonam 
Explanation: In patient with severe sensitivity to penicillin, all beta lactams except monobactams are contraindicated. Both Aztreonam and imipenem are effective for gram negative infections but as the patient is allergic to beta lactams (carbapenems show cross allergy), Aztreonam is preferred in such a patient.