A 72-year-old woman with diabetes mellitus and chronic kidney disease (Stage 3b) is admitted with a 3-day history of dysuria, frequency, and suprapubic pain. Urinalysis shows pyuria and bacteriuria. Urine culture grows a mucoid, oxidase-positive, Gram-negative rod that produces a blue-green pigment and is resistant to aminoglycosides (tobramycin MIC >16 µg/mL) but susceptible to carbapenems and fluoroquinolones. Blood cultures are negative. What is the most likely diagnosis and the most appropriate antibiotic choice?

Explanation

## Microbiological Identification **Key Point:** The combination of oxidase-positive, Gram-negative rod, mucoid colony morphology, and blue-green pigment (pyocyanin) is pathognomonic for *Pseudomonas aeruginosa*. | Identifying Feature | Significance | |---|---| | Oxidase-positive | Distinguishes *Pseudomonas* from Enterobacteriaceae | | Blue-green pigment | Pyocyanin production (characteristic of *P. aeruginosa*) | | Mucoid morphology | Indicates alginate production; often seen in chronic/biofilm infections | | Gram-negative rod | Confirms bacterial classification | ## Antibiotic Susceptibility Pattern **High-Yield:** This isolate shows **aminoglycoside resistance** (tobramycin MIC >16 µg/mL), which is a critical finding that eliminates combination therapy with aminoglycosides. This resistance pattern suggests: - Upregulation of efflux pumps (mexAB-oprM system) - Reduced aminoglycoside uptake - Possible *P. aeruginosa* with altered membrane permeability **Preserved susceptibility** to carbapenems and fluoroquinolones indicates this is NOT a carbapenem-resistant *Pseudomonas* (CRPA). ## Treatment Rationale for Urinary Tract Infection **Clinical Pearl:** Unlike respiratory or bloodstream *Pseudomonas* infections (which require dual therapy), uncomplicated *Pseudomonas* urinary tract infection in a non-bacteremic patient can be treated with monotherapy, provided: 1. The organism is susceptible to the chosen agent 2. Adequate urinary concentrations are achieved 3. The patient is not immunocompromised or septic **Key Point:** For *Pseudomonas* UTI with aminoglycoside resistance: - **Carbapenems (meropenem, imipenem)** are the preferred agents because they achieve excellent urinary concentrations and maintain activity against resistant strains - **Fluoroquinolones** are acceptable alternatives for oral step-down or mild infections, but IV carbapenem is preferred for initial therapy in hospitalized elderly patients with comorbidities **Mnemonic: CARB-UTI** — *Carbapenem* for *Pseudomonas* in *Urinary Tract Infection* when aminoglycoside-resistant ## Dosing for Renal Impairment **Warning:** This patient has Stage 3b CKD (eGFR 30–44 mL/min/1.73m²). Meropenem dosing must be adjusted: - **Standard:** 1 g IV Q8H - **eGFR 30–44:** 500 mg IV Q8H (or 1 g IV Q12H) - **eGFR <30:** 500 mg IV Q12H The option specifies 500 mg IV Q8H, which is appropriate for this patient's renal function. ## Why Monotherapy Is Appropriate Here 1. **Non-bacteremic:** Blood cultures negative → no systemic sepsis 2. **Localized infection:** UTI confined to urinary tract 3. **Adequate renal function for drug clearance:** eGFR 30–44 allows carbapenem use with dose adjustment 4. **Susceptible to carbapenem:** Organism is not CRPA ## Duration **High-Yield:** *Pseudomonas* UTI typically requires 10–14 days of therapy (longer than *E. coli* UTI, which is 3–5 days) due to the organism's virulence and biofilm-forming capacity.