Urinary Tract Infections & Pyelonephritis

Urinary tract infection (UTI) is microbial colonisation of urine plus invasion of any part of the urinary tract, from urethra to renal cortex. For NEET PG, the high-yield axis runs from ascending infection pathogenesis through the morphology of acute and chronic pyelonephritis to the special "named-body" entities — xanthogranulomatous pyelonephritis and malakoplakia with Michaelis-Gutmann bodies.

Definitions & classification

UTI is broadly split by anatomical level and by host status:

• Lower UTI — urethritis, cystitis, prostatitis. Symptoms: dysuria, frequency, urgency, suprapubic pain; no fever usually.
• Upper UTI — pyelonephritis (renal pelvis + parenchyma) and ureteritis. Symptoms: fever, flank/loin pain, costovertebral angle (CVA) tenderness ± lower-tract symptoms.
• Uncomplicated UTI — normal tract, non-pregnant, immunocompetent woman.
• Complicated UTI — structural/functional abnormality, obstruction, catheter, pregnancy, diabetes, immunosuppression, male sex, or hospital acquisition.
• Asymptomatic bacteriuria — significant bacteriuria without symptoms; treated only in pregnancy and before urological instrumentation.

High-yield: "Significant bacteriuria" on a clean-catch midstream sample = ≥10⁵ CFU/mL (Kass criterion). In a symptomatic woman with pyuria, ≥10² CFU/mL of a coliform is significant; any growth from a suprapubic aspirate is significant.

Etiology — the organisms

Most UTIs are caused by gut commensals ascending the urethra. The single most important organism across all settings is uropathogenic Escherichia coli (UPEC).

Setting	Most common organism	Notable points
Community-acquired, uncomplicated	E. coli (~80%)	UPEC with P-fimbriae
Young sexually active women	Staphylococcus saprophyticus	2nd commonest in this group; coagulase-negative, novobiocin-resistant
Hospital/catheter-associated	E. coli, Klebsiella, Proteus, Pseudomonas, Enterococcus, Candida	Biofilm on catheter
Struvite (staghorn) stones	Urease producers: Proteus, Klebsiella, Staphylococcus saprophyticus, Ureaplasma	Alkaline urine → Mg-ammonium-phosphate
Recurrent/structural disease	Proteus	Swarming motility; urease splits urea → struvite
Honeymoon cystitis	E. coli	Post-coital

High-yield: Proteus and other urease-positive organisms alkalinise urine and produce struvite (triple-phosphate) staghorn calculi. Remember the urease producers as PUNCH KS — Proteus, Ureaplasma, Nocardia, Cryptococcus, H. pylori, Klebsiella, Staph saprophyticus.

Pathogenesis — ascending vs haematogenous

There are two routes; the ascending route dominates.

Ascending route (commonest): Colonisation of distal urethra/introitus by faecal flora → ascent to bladder (aided by instrumentation, sexual intercourse, short female urethra) → colonisation of bladder = cystitis → with vesicoureteric reflux (VUR) and intrarenal reflux, bacteria ascend the ureter to the pelvis → entry into renal parenchyma = pyelonephritis.

Haematogenous route: Seeding of the kidney during septicaemia or infective endocarditis. Typical organisms: Staphylococcus aureus and Candida. This route favours cortical involvement and multiple abscesses.

Key host/anatomical predisposing factors:

1. Female sex — short urethra, proximity to anus.
2. Urinary obstruction — stones, BPH, tumour, stricture → stasis.
3. Vesicoureteric reflux — incompetent vesicoureteric valve; the prime reason childhood UTI scars the kidney.
4. Catheterisation / instrumentation.
5. Pregnancy — progesterone-induced ureteric dilatation + stasis (4–6% develop asymptomatic bacteriuria; up to 30% of these progress to pyelonephritis if untreated).
6. Diabetes mellitus — predisposes to pyelonephritis, emphysematous infection, and papillary necrosis.
7. Immunosuppression.

High-yield: Intrarenal reflux occurs preferentially at the poles of the kidney because the compound (flat-topped, concave) papillae there permit reflux, whereas mid-zone simple (convex) papillae resist it. Hence childhood reflux scars are polar.

Virulence factors of UPEC worth remembering: P-fimbriae (pap pili) binding Gal-Gal moieties on uroepithelium (associated with pyelonephritis), type-1 fimbriae (FimH, mannose-sensitive, bladder adhesion), haemolysin, aerobactin (iron acquisition), and K-antigen capsule.

Acute pyelonephritis — morphology

Acute pyelonephritis is acute suppurative (neutrophilic) inflammation of the kidney, the renal pathology hallmark of upper UTI.

Gross: Discrete yellowish, raised abscesses on the cortical surface; on cut section, yellow streaks radiate from medulla to cortex along the tubules (suppuration tracking up the collecting system). With ascending infection, the pelvicalyceal mucosa is congested/inflamed first; with haematogenous infection, the cortex is hit first.

Microscopy: Patchy interstitial neutrophilic infiltrate, neutrophil casts within tubular lumina (intratubular neutrophils — almost diagnostic of active pyelonephritis), tubular necrosis. Glomeruli are characteristically spared early.

Three complications/special forms:

• Papillary necrosis — ischaemic + suppurative necrosis of the renal papillae; grey-white to yellow necrotic tips. Causes mnemonic POSTCARD: Pyelonephritis, Obstruction, Sickle cell, Tuberculosis, Cirrhosis/Chronic alcoholism, Analgesic abuse (phenacetin/NSAID), Renal vein thrombosis, Diabetes mellitus. Especially in diabetics and analgesic nephropathy (where it is often bilateral).
• Pyonephrosis — total or near-total obstruction so pus fills and distends the pelvicalyceal system.
• Perinephric abscess — extension through the renal capsule into perinephric fat.

High-yield: White-cell (neutrophil/leucocyte) casts in urine localise infection to the renal parenchyma (pyelonephritis) rather than the bladder. Intratubular neutrophils on biopsy = active pyelonephritis.

Emphysematous pyelonephritis — necrotising infection with gas in renal parenchyma, almost exclusively in diabetics, usually E. coli or Klebsiella. CT shows intraparenchymal gas; it is a surgical emergency frequently needing nephrectomy.

Chronic pyelonephritis — the scarring kidney

Chronic pyelonephritis (CPN) is chronic tubulointerstitial inflammation and scarring involving the calyces and pelvis, an important cause of end-stage kidney disease. It is defined by the combination of coarse, asymmetric corticomedullary scars overlying dilated, blunted, deformed calyces.

Two pathogenic forms:

Feature	Reflux nephropathy (chronic reflux-associated)	Chronic obstructive pyelonephritis
Mechanism	VUR + intrarenal reflux ± infection in childhood	Recurrent infection superimposed on obstruction
Laterality	Often bilateral if reflux bilateral; can be unilateral	Depends on level of obstruction
Scars	Polar, overlying deformed calyces	Diffuse
Commonest cause of CPN	Yes — reflux nephropathy is the commonest form	Less common

Gross morphology: Asymmetrically contracted scarred kidney with coarse, irregular, broad scars (contrast with the fine, diffuse granular surface of benign nephrosclerosis). The hallmark is a scar overlying a deformed, dilated, blunted calyx — diagnostic of CPN and best demonstrated radiologically.

Microscopy:

• Tubular atrophy + dilatation; dilated tubules filled with eosinophilic pink colloid-like casts giving a "thyroidisation" appearance (resembles thyroid follicles) — a classic NEET PG image.
• Chronic interstitial inflammation and fibrosis.
• Periglomerular fibrosis; later, secondary focal segmental glomerulosclerosis (FSGS) from hyperfiltration — explains the proteinuria that develops late.
• Arterio/arteriolosclerosis.

High-yield: "Thyroidisation" of tubules (atrophic tubules with colloid casts) + coarse scars over blunted calyces = chronic pyelonephritis. Secondary FSGS in a scarred kidney signals progression and proteinuria.

High-yield: Differentiating coarse vs fine scarring is a favourite question — chronic pyelonephritis = coarse, irregular, asymmetric scars over deformed calyces; benign nephrosclerosis (hypertension) = fine, diffuse, symmetric granularity; TB = irregular but with calcification, "putty kidney".

Xanthogranulomatous pyelonephritis (XGP)

A rare, destructive chronic infection, usually with Proteus or E. coli, classically associated with staghorn calculi and obstruction in a middle-aged diabetic/female.

• Gross: Enlarged kidney with yellow-orange nodular masses replacing parenchyma; can mimic renal cell carcinoma clinically and radiologically.
• Microscopy: Sheets of lipid-laden foamy macrophages (xanthoma cells) admixed with lymphocytes, plasma cells, neutrophils, and giant cells.
• Management: Often requires nephrectomy; the great trap is misdiagnosis as malignancy.

High-yield: XGP = yellow nodular masses + foamy macrophages + staghorn calculus, masquerades as renal cell carcinoma. Associated organism: Proteus mirabilis.

Malakoplakia & Michaelis-Gutmann bodies

Malakoplakia ("soft plaque") is a rare chronic granulomatous reaction, most often in the bladder (also ureter/kidney), linked to chronic E. coli (coliform) infection and a defect in macrophage phagolysosomal killing (failure to degrade phagocytosed bacteria). Seen in immunosuppressed patients (transplant recipients, chronic disease).

• Gross: Soft, yellow-tan mucosal plaques in the bladder.
• Microscopy: Sheets of large foamy macrophages — von Hansemann cells / histiocytes — containing partially digested bacteria, plus laminated mineralised concretions.
• Michaelis-Gutmann bodies (MG bodies): Concentrically laminated, basophilic, target/owl-eye intracytoplasmic (and extracellular) concretions formed by calcium phosphate deposition on undigested bacterial debris. They are PAS-positive, and stain positively for calcium (von Kossa) and iron (Prussian blue / Perls) — this triple-staining is the classic exam clincher.

High-yield: Michaelis-Gutmann bodies = pathognomonic of malakoplakia; targetoid laminated bodies that are PAS + von Kossa (Ca) + Prussian blue (Fe) positive, within von Hansemann macrophages, caused by defective lysosomal killing of E. coli.

Clinical features & quick localisation

• Cystitis (lower UTI): dysuria, frequency, urgency, suprapubic discomfort, cloudy/foul urine, no systemic signs.
• Acute pyelonephritis (upper UTI): abrupt fever with chills/rigors, flank pain, CVA (Murphy's renal punch) tenderness, nausea/vomiting, ± lower-tract symptoms; may progress to sepsis.
• Chronic pyelonephritis: often insidious/silent; presents late with hypertension, polyuria, and progressive renal failure, sometimes incidentally on imaging.

In neonates/infants, UTI is non-specific — fever, poor feeding, failure to thrive — so a high index of suspicion is mandatory, and a first febrile UTI in a child warrants evaluation for reflux.

Diagnosis & investigation of choice

Urinalysis & microscopy is the bedside workhorse:

• Pyuria (>10 WBC/µL or >5/HPF), bacteriuria.
• Nitrite-positive dipstick (Enterobacteriaceae reduce nitrate → nitrite; absent with Enterococcus, Staph saprophyticus, Pseudomonas which don't reduce nitrate).
• Leucocyte esterase-positive (marker of pyuria).
• WBC casts → renal parenchymal infection (pyelonephritis).

Urine culture is the investigation of choice / gold standard for confirming UTI and guiding therapy (apply Kass count thresholds above).

Sterile pyuria (pyuria with negative routine culture) is a classic exam cue → think tuberculosis of the urinary tract, Chlamydia/gonococcal urethritis, partially treated UTI, or analgesic nephropathy.

Imaging (reserved for complicated/recurrent UTI, suspected obstruction, or children):

• Ultrasound — first line; detects hydronephrosis, abscess, stones, pyonephrosis.
• CT — best for emphysematous pyelonephritis, perinephric abscess, XGP, papillary necrosis.
• Micturating cystourethrogram (MCUG/VCUG) — investigation of choice to demonstrate and grade vesicoureteric reflux in children.
• DMSA scan — most sensitive for detecting renal cortical scarring (and acute pyelonephritic defects).

High-yield: MCUG = diagnosis/grading of VUR; DMSA = renal scar detection. Sterile pyuria → think genitourinary TB.

Management & drug of choice

Acute uncomplicated cystitis (women): Short-course oral therapy — nitrofurantoin 5 days, fosfomycin single dose, or trimethoprim-sulfamethoxazole (where local resistance <20%). Avoid nitrofurantoin if CrCl is low or in pyelonephritis (poor tissue levels).

Acute pyelonephritis: Empirical fluoroquinolone (ciprofloxacin) or a third-generation cephalosporin; hospitalise and give IV (ceftriaxone, or aminoglycoside ± ampicillin, or a carbapenem for resistant organisms) if vomiting/sepsis/pregnancy, then de-escalate by culture. Treat 7–14 days. Relieve obstruction (drainage of pyonephrosis/abscess) — antibiotics alone will fail in obstructed pus.

Asymptomatic bacteriuria: Treat only in pregnancy and pre-instrumentation. In pregnancy, safe choices include nitrofurantoin (avoid near term), cephalexin, and amoxicillin; avoid fluoroquinolones and co-trimoxazole.

XGP / non-functioning infected kidney: Nephrectomy. Emphysematous pyelonephritis: Resuscitate + IV antibiotics + drainage; nephrectomy if no response. Recurrent UTI: Behavioural measures, post-coital or low-dose prophylaxis; correct underlying VUR/obstruction.

Complications

• Sepsis / septic shock (especially obstructed infection — urosepsis).
• Renal/perinephric abscess, pyonephrosis.
• Papillary necrosis (diabetes, analgesics, sickle cell).
• Emphysematous pyelonephritis.
• Chronic pyelonephritis → hypertension and chronic kidney disease/ESRD (reflux nephropathy is a leading paediatric cause of CKD).
• Struvite staghorn calculi with urease producers.
• In pregnancy: preterm labour, low birth weight.

Key differentials

Entity	Distinguishing clue
Acute interstitial nephritis (drug-induced)	Eosinophils + eosinophiluria, drug history, no bacteriuria
Genitourinary tuberculosis	Sterile pyuria, calcification/"putty kidney", caseating granulomas
Renal cell carcinoma	Mass; mimicked by XGP (foamy macrophages distinguish)
Glomerulonephritis	RBC casts, dysmorphic RBCs, proteinuria — not WBC casts
Analgesic nephropathy	Bilateral papillary necrosis, NSAID/phenacetin history
Benign nephrosclerosis	Fine symmetric scarring (vs coarse in CPN)

Recently asked / exam angle

• Michaelis-Gutmann bodies → malakoplakia; recall the triple stain (PAS, von Kossa, Prussian blue) and von Hansemann macrophages. Repeatedly tested one-liner.
• Thyroidisation of tubules as the histologic image of chronic pyelonephritis.
• "Coarse scar over a deformed/blunted calyx" — single best diagnostic feature of chronic pyelonephritis.
• XGP — yellow masses, foamy cells, staghorn stone, Proteus, mimics RCC.
• Investigation pairing: MCUG for VUR, DMSA for scars, CT for emphysematous PN/abscess.
• Urease producers and struvite/staghorn calculi; Proteus swarming.
• Significant bacteriuria cut-offs (Kass: ≥10⁵; symptomatic ≥10²; suprapubic any growth).
• Sterile pyuria → TB.
• Papillary necrosis POSTCARD causes; bilateral in analgesic abuse and diabetes.
• WBC casts localise to kidney; S. saprophyticus in young women; S. aureus via haematogenous route.

Rapid revision

1. Commonest UTI organism overall = uropathogenic E. coli (P-fimbriae for pyelonephritis).
2. Young sexually active women → 2nd commonest = Staph saprophyticus (novobiocin-resistant, coagulase-negative).
3. Kass count = ≥10⁵ CFU/mL; symptomatic woman ≥10²; suprapubic aspirate = any growth.
4. WBC casts = parenchymal (pyelonephritis); RBC casts = glomerular.
5. Acute pyelonephritis = neutrophilic suppuration, intratubular neutrophils, yellow cortical abscesses, glomeruli spared.
6. Papillary necrosis — POSTCARD (esp. diabetes, analgesics, sickle cell); analgesic type is bilateral.
7. Emphysematous pyelonephritis — gas-forming, diabetics, E. coli/Klebsiella, often needs nephrectomy.
8. Chronic pyelonephritis = coarse scars over blunted calyces + tubular thyroidisation + late secondary FSGS.
9. Reflux nephropathy is the commonest form of CPN; polar scars from intrarenal reflux at compound papillae.
10. XGP = foamy macrophages + staghorn stone + Proteus; mimics RCC; treat by nephrectomy.
11. Malakoplakia → Michaelis-Gutmann bodies (PAS+, von Kossa+, Prussian blue+) in von Hansemann macrophages; defective lysosomal killing of E. coli.
12. Investigation pearls: culture = gold standard; MCUG → VUR, DMSA → scars; sterile pyuria → TB; treat asymptomatic bacteriuria only in pregnancy and pre-instrumentation.