A 42-year-old Caucasian male presents to your office with hematuria and right flank pain. He has no history of renal dialysis but has a history of recurrent urinary tract infections. You order an intravenous pyelogram, which reveals multiple cysts of the collecting ducts in the medulla. What is the most likely diagnosis?

Acquired polycystic kidney disease

A 40-year-old man comes to the physician because of a 4-week history of generalized weakness. He also reports increased urination and thirst. He has type 2 diabetes mellitus and chronic kidney disease. His only medication is metformin. Serum studies show: Na+ 134 mEq/L Cl- 110 mEq/L K+ 5.6 mEq/L HCO3- 19 mEq/L Glucose 135 mg/dL Creatinine 1.6 mg/dL Urine pH is 5.1. Which of the following is the most likely underlying cause of this patient's symptoms?

Decreased serum aldosterone levels

Impaired HCO3- reabsorption in the proximal tubule

Decreased serum cortisol levels

Impaired H+ secretion in the distal tubule

A 45-year-old woman presents to the office with a complaint of generalized weakness that has been getting worse over the last few months. She says that she just does not have the energy for her daily activities. She gets winded quite easily when she takes her dog for a walk in the evening. She says that her mood is generally ok and she gets together with her friends every weekend. She works as a teacher at a local elementary school and used to have frequent headaches while at work. Her husband is a commercial pilot and is frequently away for extended periods of time. Her only son is a sophomore in college and visits her every other week. She has had issues in the past with hypertension, but her blood pressure is currently well-controlled because she is compliant with her medication. She is currently taking atorvastatin and lisinopril. The blood pressure is 130/80 mm Hg, the pulse is 90/min, the temperature is 36.7°C (98.0°F), and the respirations are 16/min. On examination, she appears slightly pale and lethargic. Her ECG today is normal and recent lab work shows the following: Serum creatinine 1.5 mg/dL Estimated GFR 37.6 mL/min Hemoglobin (Hb%) 9 mg/dL Mean corpuscular hemoglobin (MCH) 27 pg Mean corpuscular hemoglobin concentration (MCHC) 36 g/dL Mean corpuscular volume (MCV) 85 fL Reticulocyte count 0.1% Erythrocyte count 2.5 million/mm3 Serum iron 160 μg/dL Serum ferritin 150 ng/mL Total iron binding capacity 105 μg/dL Serum vitamin B12 254 pg/mL Serum folic acid 18 ng/mL Thyroid stimulating hormone 3.5 μU/mL Which of the following will most likely help her?

Start vitamin B12 with folic acid.

A 70-year-old man is brought to the emergency department by his wife because of lethargy, confusion, and nausea for the past 2 days. He has previously been healthy and has no past medical history. His only medications are a daily multivitamin and acetaminophen, which he takes daily for hip pain. Vital signs are within normal limits. He is disoriented to place and time but recognizes his wife. The remainder of his physical examination shows no abnormalities. Laboratory studies show a hemoglobin concentration of 9.1 g/dL, a serum calcium concentration of 14.7 mg/dL, and a serum creatinine of 2.2 mg/dL (previously 0.9 mg/dL). Which of the following is the most likely underlying mechanism of this patient's condition?

Overproliferation of plasma cells

Increased serum levels of 1,25-hydroxyvitamin D

Excessive consumption of calcium

Excess PTH secretion from parathyroid glands

Renal cystic diseases for USMLE Step 3: High-Yield Pathology Lessons

ADPKD - Adult Polycystic Mayhem

Genetics: Autosomal dominant. Mutation in PKD1 (chromosome 16, ~85%) or PKD2 (chromosome 4, ~15%), coding for polycystin-1 or -2.
Pathophysiology: Numerous cysts in all parts of the nephron cause bilateral, massive kidney enlargement.
Clinical Triad: Presents in adulthood (40s-50s) with flank pain, hematuria, and hypertension (early, due to RAAS activation), leading to progressive renal failure.
Extra-renal Manifestations:
- Hepatic cysts (most common)
- Berry aneurysms (Circle of Willis)
- Mitral valve prolapse
- Colonic diverticula

⭐ Exam Favorite: Rupture of a berry aneurysm is a major cause of death, leading to subarachnoid hemorrhage. Screen symptomatic patients or those with a family history of aneurysms.

📌 Mnemonic: Pain, Kidneys massive, Diverticula/aneurysms.

ADPKD vs. ARPKD: Kidney and Nephron Manifestations

ARPKD - Recessive Renal Rumpus

Genetics: Autosomal recessive mutation in the PKHD1 gene (chromosome 6), affecting the fibrocystin protein.
Pathology:
- Bilaterally enlarged, echogenic kidneys with a smooth surface.
- Cysts are elongated, arising from collecting ducts and arranged radially.
Clinical Presentation:
- Presents in infancy or in utero.
- Severe cases may show Potter sequence (oligohydramnios, pulmonary hypoplasia).
- Systemic hypertension and progressive renal insufficiency.

⭐ High-Yield: ARPKD is invariably associated with congenital hepatic fibrosis, which can lead to portal hypertension.

Ultrasound of ARPKD: enlarged echogenic kidneys

📌 Mnemonic: Autosomal Recessive Potter Kidney Disease (links inheritance, common severe outcome, and organ).

Medullary Mix-up - Spongy vs. Cystic

Feature	Medullary Sponge Kidney (MSK)	Medullary Cystic Kidney Disease (MCKD)
Etiology	Sporadic, developmental anomaly	Autosomal Dominant (AD)
Pathology	Dilated papillary collecting ducts	Cysts at corticomedullary junction, fibrosis
Kidney Size	Normal or enlarged	Shrunken kidneys
Key Finding	Incidental; recurrent kidney stones	Salt wasting, progressive renal failure
Prognosis	Benign	Leads to end-stage renal disease (ESRD)

⭐ Key Distinction: MCKD is a progressive disease causing tubulointerstitial fibrosis, leading to shrunken kidneys and significant salt wasting, unlike the benign, normal-sized kidneys in MSK.

Acquired Cysts - Dialysis & Danger

Develops in patients with end-stage renal disease, strongly associated with long-term dialysis (especially >3-5 years).
Native kidneys are often shrunken/atrophic but develop multiple bilateral cortical and medullary cysts.
⚠️ High risk of malignant transformation. The primary danger is a significantly increased incidence of renal cell carcinoma (RCC), which can be bilateral and multifocal.

⭐ The risk of developing RCC in dialysis patients with acquired cystic disease is up to 7% over 10 years, a substantial increase over the general population.

Autosomal Dominant Polycystic Kidney Disease (ADPKD) results from PKD1/PKD2 mutations, causing bilaterally enlarged kidneys, hypertension, and pain in adults.

Key ADPKD extrarenal findings include berry aneurysms, hepatic cysts, and mitral valve prolapse.

Autosomal Recessive (ARPKD) presents in infancy, often with Potter sequence, and is linked to congenital hepatic fibrosis.

Medullary Cystic Kidney Disease causes tubulointerstitial fibrosis, leading to shrunken kidneys and eventual renal failure.

Acquired cystic disease occurs in chronic dialysis patients, increasing their risk for renal cell carcinoma.

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