Empiric antibiotic therapy with cefepime, levofloxacin, and gentamicin is the most appropriate regimen for this patient. She presents with severe community-acquired pneumonia (CAP) requiring ICU treatment and, therefore, requires appropriate antibiotics for this setting (typically, a β-lactam plus either azithromycin or a fluoroquinolone, such as cefepime, levofloxacin, or ciprofloxacin). Patients at increased risk for Pseudomonas aeruginosa infection include those with structural lung disease (such as bronchiectasis) and recent or frequent glucocorticoid and antibiotic use. Because this patient has bronchiectasis and recent glucocorticoid use, she is at increased risk for Pseudomonas infection and should be treated with a β-lactam with antipseudomonal activity (such as cefepime) as part of her regimen with the addition of another antipseudomonal agent, such as gentamicin.

A β-lactam in combination with a macrolide or doxycycline, such as ceftriaxone and azithromycin, is an appropriate empiric regimen for patients requiring inpatient treatment in a medical ward setting. However, it does not provide adequate coverage for P. aeruginosa infection and thus would not be an appropriate regimen for this patient. A respiratory fluoroquinolone plus aztreonam is an alternative regimen to empiric treatment with a β-lactam plus either azithromycin or a fluoroquinolone in patients requiring treatment in the ICU for CAP, particularly for patients who are allergic to penicillin. However, this regimen does not provide adequately robust coverage against P. aeruginosa.

The combination of ceftriaxone and levofloxacin provides some coverage against P. aeruginosa but would not be the best choice for an antibiotic regimen targeting suspected P. aeruginosa CAP because of ceftriaxone's inadequate coverage against P. aeruginosa.

Meropenem, a broad-spectrum carbapenem β-lactam antibiotic, would be an appropriate component of an empiric regimen for treatment of possible Pseudomonas infection only if given in combination with a respiratory fluoroquinolone or another agent providing pseudomonal coverage. Meropenem monotherapy would not provide adequate coverage against Pseudomonas infection and thus would not be an appropriate empiric treatment regimen for this patient.