A 32-year-old woman comes to the physician because of flank pain, myalgia, and reddish discoloration of her urine for the past 2 days. One week ago, she had a fever and a sore throat and was prescribed antibiotics. She is otherwise healthy and has no history of serious illness. Her temperature is 37.9°C (100.2°F), pulse is 70/min, and blood pressure is 128/75 mm Hg. Physical examination shows a soft abdomen and no costovertebral angle tenderness. Examination of the mouth and pharynx shows no abnormalities. There is a faint maculopapular rash over the trunk and extremities. Serum creatinine is 2.4 mg/dL. Urinalysis shows: Protein 2+ Blood 2+ RBC 20–30/hpf WBC 12/hpf Bacteria none Which of the following is the most likely diagnosis?

Allergic interstitial nephritis

Crystal-induced acute kidney injury

A 57-year-old man comes to the emergency department because of pain in the sides of his abdomen and blood-tinged urine since the previous night. Over the last 2 days, he has also had progressive malaise, myalgia, and a generalized itchy rash. He has a history of gastroesophageal reflux that did not respond to ranitidine but has improved since taking pantoprazole 2 months ago. He occasionally takes acetaminophen for back pain. His vital signs are within normal limits. Examination shows a generalized, diffuse maculopapular rash. The remainder of the examination shows no abnormalities. Laboratory studies show: Hemoglobin 13 g/dL Leukocyte count 7,800/mm3 Serum Na+ 140 mEq/L Cl- 105 mEq/L K+ 4.6 mEq/L HCO3- 25 mEq/L Glucose 102 mg/dL Creatinine 4.1 mg/dL Renal ultrasonography shows no abnormalities. Which of the following findings is most likely to be observed in this patient?

Elevated levels of eosinophils in urine

Mesangial IgA deposits on renal biopsy

Urinary crystals on brightfield microscopy

Crescent-shape extracapillary cell proliferation

Papillary calcifications on CT imaging

A 34-year-old female presents to her primary care physician with complaints of fevers, nausea/vomiting, and severe left flank pain that has developed over the past several hours. She denies any prior episodes similar to her current presentation. Physical examination is significant for a body temperature of 39.1 C and costovertebral angle tenderness. A urinalysis and urine microscopy are ordered. Which of the following findings on kidney histology would be expected in this patient?

Neutrophils filling the lumens of the renal tubules

Enlarged, hypercellular glomeruli with 'wire-looping' of capillaries

Interstitial fibrosis and lymphocytic infiltrate

Thickening of the capillaries and glomerular basement membrane

A 36-year-old man comes to the physician for a 4-week history of swollen legs. He has difficulty putting on socks because of the swelling. Two years ago, he was diagnosed with sleep apnea. He takes no medications. He emigrated from Guatemala with his family when he was a child. He is 171 cm (5 ft 6 in) tall and weighs 115 kg (253 lb); BMI is 39 kg/m2. His pulse is 91/min and blood pressure is 135/82 mm Hg. Examination shows periorbital and bilateral lower extremity edema. Serum Albumin 3.1 g/dL Total cholesterol 312 mg/dL Urine Blood negative Protein +4 RBC 1-2/hpf RBC cast negative Fatty casts numerous A renal biopsy is obtained. Which of the following is most likely to be seen under light microscopy of the patient's renal biopsy specimen?

Segmental sclerosis of the glomeruli

Diffuse thickening of glomerular capillaries

Amyloid deposition in the mesangium

Eosinophilic nodules within the glomeruli

Fibrin crescents within the glomerular space

A 15-year-old Caucasian male is brought to his pediatrician by his parents, who note the development of a tremor in their child. Urine and serum analysis reveal elevated levels of copper. Which of the following clinical manifestations would the physician most expect to see in this patient?

Hepatocytes that stain with Prussian blue

Kidney diseases for USMLE Step 1: High-Yield Pathology Lessons

Glomerular Diseases - The Great Filter Failure

Core Pathophysiology: Damage to the glomerular filtration barrier (endothelium, GBM, podocytes) dictates the clinical syndrome.

Nephrotic Syndrome: Podocyte damage leads to massive protein loss.
- Features: Pitting edema, hypoalbuminemia, hyperlipidemia, lipiduria (oval fat bodies).
- Key Causes: Minimal Change Disease (kids), FSGS, Membranous Nephropathy, Diabetic Nephropathy.
Nephritic Syndrome: Inflammation-driven damage.
- Features: Hypertension, oliguria, hematuria (RBC casts are pathognomonic), mild proteinuria.
- Key Causes: Post-streptococcal GN, IgA Nephropathy (Berger disease), Lupus Nephritis.

⭐ Exam Favorite: Subepithelial immune complex "humps" on electron microscopy are classic for Post-Streptococcal Glomerulonephritis.

Post-streptococcal glomerulonephritis: Light, IF, EM

Tubulointerstitial Disease - When Pipes Go Bad

Inflammation or fibrosis of tubules & interstitium, initially sparing the glomeruli.
Acute Interstitial Nephritis (AIN):
- Type IV hypersensitivity, often drug-induced (NSAIDs, penicillins, PPIs).
- Presents with fever, rash, arthralgias, and acute kidney injury (↑ creatinine).
- Urinalysis: Sterile pyuria (WBC casts), eosinophiluria.
- 📌 Mnemonic (Causes): The "P" drugs - Pee (diuretics), Pain-free (NSAIDs), Penicillins, PPIs.
Chronic Interstitial Nephritis:
- Prolonged inflammation leads to interstitial fibrosis, tubular atrophy, & progressive CKD.
- Key causes: Analgesic abuse, lead, reflux nephropathy.

⭐ The classic triad of fever, rash, and eosinophilia for drug-induced AIN is present in only 10-15% of patients. Suspect it in any patient with AKI after starting a new medication.

Acute interstitial nephritis with eosinophilic infiltrates

Vascular & Cystic Diseases - Plumbing & Pockets

Hypertensive Nephrosclerosis: Arteriolosclerosis (hyaline or hyperplastic "onion-skinning") → glomerulosclerosis & atrophy.
Renal Artery Stenosis (RAS): Due to atherosclerosis or fibromuscular dysplasia. Causes ↑renin & secondary hypertension.
Autosomal Dominant Polycystic Kidney Disease (ADPKD): Adult onset. Mutations in PKD1 or PKD2. Bilateral, massive cystic kidneys.
Autosomal Recessive (ARPKD): Infancy/childhood. PKHD1 mutation. Associated with hepatic fibrosis.

ADPKD vs. ARPKD: Kidney Morphology & Clinical Features

⭐ Patients with ADPKD have an increased risk of intracranial berry aneurysms.

Renal Neoplasia - Unwanted Growths

Renal Cell Carcinoma (RCC): Most common renal malignancy in adults. Classic triad (hematuria, flank pain, mass) is uncommon. Associated with paraneoplastic syndromes (EPO, PTHrP). Clear cell type linked to VHL gene mutation.
Wilms Tumor (Nephroblastoma): Most common renal tumor in children (age 2-5). Associated with WAGR syndrome.
Benign Tumors:
- Angiomyolipoma: Hamartoma; linked to tuberous sclerosis.
- Oncocytoma: Eosinophilic cells; may have a central stellate scar.

Clear Cell Renal Carcinoma Histopathology Diagram

⭐ RCC has a propensity to invade the renal vein and inferior vena cava (IVC), leading to hematogenous spread.

Nephrotic syndrome is marked by >3.5 g/day proteinuria and edema, while nephritic syndrome features hematuria and RBC casts.

Minimal change disease is the leading cause of nephrotic syndrome in children, showing podocyte effacement.

Membranous nephropathy is the most common primary cause in adults, with a characteristic "spike and dome" appearance.

Post-streptococcal GN presents as a nephritic syndrome with subepithelial "humps" after infection.

Acute Tubular Necrosis (ATN) is the #1 cause of AKI, identified by muddy brown casts.

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Kidney diseases