Pyelonephritis
The most likely diagnosis is C. Pyelonephritis, based on the classic triad of fever, flank pain (left back pain), and dysuria with associated nausea.
Clinical Reasoning
This patient presents with the hallmark features of acute pyelonephritis:
- Fever (100.4°F) - Pyelonephritis typically presents with fever >38°C (100.4°F), representing systemic infection of the kidney 1, 2
- Flank/back pain - Costovertebral angle tenderness or flank pain is nearly universal in pyelonephritis and distinguishes it from lower tract infections 2, 3
- Nausea - Systemic symptoms including nausea and vomiting are characteristic of upper urinary tract infection 1, 2
- Dysuria - While bladder symptoms may be present, they can be absent in up to 20% of pyelonephritis cases 2
Why Not the Other Options
Uncomplicated UTI (cystitis) is excluded because:
- Cystitis presents with dysuria, frequency, and urgency without fever 3, 4
- Lower abdominal pain rather than flank pain is typical 4
- The presence of fever and back pain indicates upper tract involvement 1, 3
Bacterial vaginosis is excluded because:
- Does not cause fever, flank pain, or dysuria 5
- Primarily causes vaginal discharge and odor without systemic symptoms
Chlamydia is excluded because:
- Typically presents with dysuria and discharge but not fever or flank pain 5
- Does not cause acute systemic illness with nausea
Diagnostic Confirmation
The diagnosis should be confirmed with:
- Urinalysis showing pyuria and/or bacteriuria 1, 2
- Urine culture with antimicrobial susceptibility testing in all suspected pyelonephritis cases 1, 2
- The absence of pyuria would suggest an alternative diagnosis 3
Important Clinical Pitfall
Do not delay treatment while awaiting culture results in a patient with clinical pyelonephritis 1. Empirical antibiotic therapy should be initiated immediately based on clinical presentation, as delayed treatment can lead to renal scarring and complications 6.