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Urinary Tract Infections for the USMLE Step 3

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Urinary Tract Infections for the USMLE Step 3 Exam
  • Definition:
    • Urinary tract infections (UTIs) involve microbial invasion of the urinary tract, including the urethra, bladder, ureters, and kidneys. UTIs are categorized into lower UTI (cystitis) and upper UTI (pyelonephritis) based on the site of infection.
  • Etiology:
    • The most common pathogen causing UTIs is Escherichia coli (75-95% of cases). Other organisms include:
    • Staphylococcus saprophyticus (especially in sexually active young women).
    • Proteus mirabilis, Klebsiella pneumoniae, and Enterococcus in complicated or healthcare-associated UTIs.
    • Fungal UTIs (mainly due to Candida) occur more frequently in immunocompromised patients or those with long-term catheter use.
  • Risk Factors:
    • Female anatomy: The shorter urethra in women increases the risk of UTIs.
    • Sexual activity: Increases bacterial introduction into the urethra.
    • Postmenopausal women: Vaginal atrophy and decreased estrogen levels predispose to infection.
    • Urinary stasis: Conditions like benign prostatic hyperplasia (BPH), ureteral strictures, and vesicoureteral reflux impede urinary flow.
    • Diabetes: Impaired immune response and high glucose levels promote bacterial growth.
    • Catheterization: Indwelling catheters provide a direct route for bacterial colonization.
  • Clinical Features:
    • Lower UTI (Cystitis):
    • Dysuria: Painful or burning urination.
    • Urinary frequency and urgency: Frequent need to void, often with a strong urge.
    • Suprapubic pain: Discomfort over the bladder area.
    • Hematuria: Blood in the urine may be present.
Cystitis urine output
    • Upper UTI (Pyelonephritis):
    • Fever and chills: Suggest systemic involvement.
    • Flank pain: Pain at the costovertebral angle.
    • Nausea and vomiting: Common with more severe infection.
    • Signs of sepsis: Severe cases can present with hypotension and altered mental status.
  • Diagnosis:
    • Urinalysis:
    • Nitrites: Positive in Gram-negative infections (e.g., E. coli).
    • Leukocyte esterase: Indicates pyuria (white blood cells in the urine).
    • Microscopy: Confirms the presence of pyuria and bacteriuria.
    • Urine Culture:
    • Required for complicated or recurrent cases. A colony count ≥10⁵ CFU/mL is diagnostic.
    • Imaging:
    • Not routinely required for uncomplicated cases but may be indicated in recurrent or complicated UTIs. Renal ultrasound or CT scan is useful for assessing obstruction or abscess formation.
  • Management:
    • Uncomplicated UTI:
    • First-line antibiotics:
    • Nitrofurantoin (5 days), trimethoprim-sulfamethoxazole (TMP-SMX) (3 days), or fosfomycin (single dose).
    • Fluoroquinolones (e.g., ciprofloxacin) are reserved for resistant infections or those with contraindications to first-line treatments.
    • Symptomatic treatment: Phenazopyridine can be used for temporary relief of dysuria.
    • Complicated UTI:
    • Requires longer antibiotic courses (7-14 days), tailored to urine culture results.
    • Hospitalization and intravenous antibiotics (e.g., ceftriaxone, fluoroquinolones) may be necessary for severe cases of pyelonephritis or sepsis.
    • Pyelonephritis:
    • Empiric antibiotics (e.g., fluoroquinolones or ceftriaxone) are started while awaiting culture results.
    • Hospitalization is indicated for systemic symptoms, dehydration, or sepsis.
    • Recurrent UTI:
    • Behavioral modifications: Increased fluid intake, post-coital voiding, and avoiding spermicides.
    • Prophylactic antibiotics: Consider for patients with frequent infections.
  • Complications:
    • Acute pyelonephritis: Can lead to renal abscess or scarring.
    • Sepsis: Urosepsis, a severe complication of pyelonephritis, can lead to septic shock, particularly in older or immunocompromised patients.
    • Chronic kidney disease (CKD): Recurrent upper UTIs may cause renal scarring, leading to CKD.
Key Points
  • UTIs are primarily caused by Escherichia coli, with cystitis affecting the lower urinary tract and pyelonephritis affecting the kidneys.
  • Diagnosis relies on urinalysis (nitrites, leukocyte esterase) and urine culture, particularly in complicated cases.
  • Uncomplicated UTIs are managed with short-course antibiotics like nitrofurantoin or TMP-SMX, while complicated infections require longer treatment and may involve hospitalization.
  • Prevention of recurrent UTIs includes behavioral modifications and, in some cases, prophylactic antibiotics.
  • Pyelonephritis can lead to serious complications, including renal scarring and sepsis, necessitating prompt treatment and hospitalization in severe cases.

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