A patient with a penicillin allergy but requiring treatment for a severe infection would best be treated with which drug?

The most appropriate choice for a patient with a penicillin allergy who requires treatment for a severe infection is aztreonam. Aztreonam is a monobactam antibiotic that is structurally distinct from penicillin and its derivatives, making it a suitable option for individuals with penicillin allergies. It specifically targets gram-negative bacteria and is often used when treating infections caused by these pathogens, especially when other alternatives are limited due to allergy.

Ciprofloxacin, while effective, is a fluoroquinolone antibiotic and may not be the first choice for certain types of severe infections, specifically those that are suspected to involve resistant organisms or where specific gram-positive coverage is needed. Metronidazole is effective against anaerobic bacteria and certain protozoa, but it is not typically used for severe infections requiring broad-spectrum, empiric gram-negative coverage. Vancomycin is primarily used to treat gram-positive infections, including those caused by methicillin-resistant Staphylococcus aureus (MRSA), but it does not provide the coverage needed against certain gram-negative organisms.

In summary, aztreonam is the best choice due to its safety profile in patients with a penicillin allergy and its efficacy against many gram-negative bacteria, making it ideal for treating