urinary tract infections

Pathophysiology: UTIs occur when uropathogens (predominantly gram-negative rods) ascend the urethra to colonize the bladder (cystitis) or ascend further via ureters to the kidneys (pyelonephritis). Women are disproportionately affected due to shorter urethra and proximity to the anus. Key organisms: Escherichia coli (most common, uses type 1 fimbriae/P fimbriae for adherence), Staphylococcus saprophyticus (second most common in young sexually active women), Klebsiella pneumoniae, Proteus mirabilis (urease-producing, alkalinizes urine, associated with struvite stones), Enterococcus faecalis, and Pseudomonas aeruginosa (catheter-associated). Risk factors: female sex, sexual activity, pregnancy, urinary catheterization, diabetes, urinary tract obstruction/stones, vesicoureteral reflux, and immunosuppression.

Clinical presentation: Cystitis presents with dysuria, urinary frequency/urgency, suprapubic pain, and possibly hematuria, without systemic symptoms. Pyelonephritis presents with fever, chills, flank pain, costovertebral angle (CVA) tenderness, nausea/vomiting, and may show WBC casts on urinalysis (pathognomonic, distinguishes from cystitis). Complicated UTIs occur in patients with anatomic abnormalities, catheters, pregnancy, or immunocompromise, requiring broader workup.

Diagnosis: Urinalysis showing pyuria (>10 WBC/hpf), positive leukocyte esterase and nitrites (nitrites indicate gram-negative organisms reducing nitrates), and bacteriuria. Urine culture with >10^5 CFU/mL confirms diagnosis (lower thresholds in symptomatic patients or catheterized specimens). Imaging (renal ultrasound, CT) reserved for recurrent, complicated, or refractory cases to evaluate for obstruction or abscess.

Treatment: Uncomplicated cystitis—nitrofurantoin, trimethoprim-sulfamethoxazole (TMP-SMX), or fosfomycin. Pyelonephritis—fluoroquinolones (e.g., ciprofloxacin) or ceftriaxone, often requiring longer courses; hospitalization for severe cases or inability to tolerate oral intake. Pregnant women require safe antibiotics (avoid fluoroquinolones and TMP-SMX in certain trimesters); asymptomatic bacteriuria in pregnancy must be treated due to risk of pyelonephritis and preterm labor.

Complications: Recurrent UTIs, pyelonephritis progressing to renal abscess or sepsis, emphysematous pyelonephritis (more common in diabetics), and chronic pyelonephritis leading to renal scarring, particularly with vesicoureteral reflux in children.

High-yield associations: Proteus and struvite stones, WBC casts specific to pyelonephritis, S. saprophyticus in young women, and E. coli virulence factors (fimbriae, hemolysins).