American Board of Internal Medicine (ABIM) Certification Practice Exam

When treating CAP in an ICU patient, which antibiotic combination is advisable if the patient has a penicillin allergy?

1. Respiratory fluoroquinolone plus aztreonam

2. β-lactam plus azithromycin

3. Macrolide with respiratory fluoroquinolone

4. Antipseudomonal β-lactam only

The correct answer is: Respiratory fluoroquinolone plus aztreonam

In the management of community-acquired pneumonia (CAP) in patients admitted to the intensive care unit (ICU), particularly in those with a penicillin allergy, using a respiratory fluoroquinolone in combination with aztreonam is a well-accepted approach. Respiratory fluoroquinolones (such as levofloxacin or moxifloxacin) are effective against common pathogens in severe CAP and provide good coverage, including atypical organisms. Meanwhile, aztreonam, a monobactam antibiotic, is generally safe for patients with penicillin allergies, as it has a different structure and does not cross-react with penicillins. This combination ensures broad-spectrum coverage, addressing both typical and atypical pathogens associated with severe pneumonia, while also avoiding the use of β-lactam antibiotics, which could trigger allergic reactions in sensitive patients. Other options either include β-lactam agents or combinations that may not provide the same breadth of coverage or safety profile for someone with a penicillin allergy. Therefore, opting for a respiratory fluoroquinolone alongside aztreonam is a prudent choice for treating CAP in ICU patients with a known allergy to penicillin.