A newborn infant is resuscitated and transferred to the neonatal intensive care unit. The infant has notable limb deformities as well as low-set ears and a flattened nose. He was born at 34 weeks gestation to a healthy mother who received regular obstetric follow-up. Resuscitation was notable for difficulty maintaining oxygenation in the newborn. Despite appropriate interventions, the infant is still struggling to maintain adequate oxygenation. Which of the following is most likely the cause of this patient's symptoms?

Cystic dilation of the collecting ducts in the kidney

Failure to administer betamethasone

A 41-year-old man presents to the emergency department because of brownish discoloration of his urine for the last several days. The review of symptoms includes complaints of increasing abdominal girth, early satiety, and difficulty breathing on exertion. The past medical history includes essential hypertension for 19 years. The medication list includes lisinopril and hydrochlorothiazide. He had a right inguinal hernia repair when he was a teenager. He smokes 20–30 cigarettes daily for the last 21 years, and drinks alcohol socially. His father died of a hemorrhagic stroke at the age of 69 years. The vital signs include: temperature 37.0°C (98.6°F), blood pressure 131/88 mm Hg, and pulse 82/min. The physical examination is positive for a palpable right upper quadrant mass. The abdominal ultrasound shows multiple bilateral kidney cysts and hepatic cysts. Which of the following is the most likely diagnosis?

Autosomal dominant polycystic kidney disease

A 4-year-old male is accompanied by his mother to the pediatrician. His mother reports that over the past two weeks, the child has had intermittent low grade fevers and has been more lethargic than usual. The child’s past medical history is notable for myelomeningocele complicated by lower extremity weakness as well as bowel and bladder dysfunction. He has been hospitalized multiple times at an outside facility for recurrent urinary tract infections. The child is in the 15th percentile for both height and weight. His temperature is 100.7°F (38.2°C), blood pressure is 115/70 mmHg, pulse is 115/min, and respirations are 20/min. Physical examination is notable for costovertebral angle tenderness that is worse on the right. Which of the following would most likely be found on biopsy of this patient’s kidney?

Tubular colloid casts with diffuse lymphoplasmacytic infiltrate

Replacement of renal parenchyma with foamy histiocytes

Diffusely necrotic papillae with dystrophic calcification

Mononuclear and eosinophilic infiltrate

Destruction of the proximal tubule and medullary thick ascending limb

A child presents with rachitic changes in the limbs that are not responding to Vitamin D supplementation. Investigations reveal the following results: - Calcium: $9.5 \mathrm{mg} / \mathrm{dl}$ - Phosphorus: $1.6 \mathrm{mg} / \mathrm{dl}$ - Alkaline phosphatase (ALP): 814 IU - Serum PTH: $24.2 \mathrm{pg} / \mathrm{ml}$ - Serum electrolytes, creatinine, and blood gases: Normal. What is the most likely diagnosis?

Vitamin D-dependent rickets type 2

Vitamin D-dependent rickets type 1

Congenital renal anomalies for USMLE Step 2 CK: High-Yield Pathology Lessons

Anomalies of Number & Position - Missing or Misplaced

Renal Agenesis: Failure of kidney development.
- Unilateral: Often asymptomatic; contralateral kidney undergoes compensatory hypertrophy. Discovered incidentally.
- Bilateral: Incompatible with life. Leads to Potter sequence.

⭐ Potter Sequence: Caused by oligohydramnios. Remember 📌 POTTER: Pulmonary hypoplasia, Oligohydramnios, Twisted face, Twisted skin, Extremity defects, Renal failure (in utero).

Renal Ectopia: Kidney not in its normal location.
- Simple: Located on the proper side, but in an abnormal position (e.g., iliac fossa).
- Pelvic (Sacral): Small, malrotated kidney in the pelvis.

Potter Syndrome: Fetal development and infant features

Anomalies of Fusion - Fused & Confused

Horseshoe Kidney: The most common congenital renal fusion anomaly.
- Embryology: Typically involves fusion of the lower poles of the two kidneys while in the pelvis.
- Ascent & Entrapment: During fetal development, the fused kidney's ascent is halted by the Inferior Mesenteric Artery (IMA).
Clinical Complications:
- Ureteropelvic junction (UPJ) obstruction → hydronephrosis.
- ↑ risk of kidney stones and infections.
- ↑ risk of renal cell carcinoma (RCC).
Associated Aneuploidies:
- Turner Syndrome
- Trisomies 13, 18, and 21

⭐ While associated with complications, the majority of individuals with a horseshoe kidney are asymptomatic and diagnosed incidentally.

Polycystic Kidney Disease - A Tale of Two Cysts

Gross and MRI of Adult Polycystic Kidney Disease

A comparison of the two major forms of polycystic kidney disease:

Feature	Autosomal Dominant (ADPKD)	Autosomal Recessive (ARPKD)
Inheritance	Autosomal Dominant	Autosomal Recessive
Gene Defect	PKD1 (85%), PKD2	PKHD1 (Fibrocystin)
Onset	Adulthood (20-40 yrs)	Infancy / In utero
Presentation	Hypertension, hematuria, flank pain, palpable bilateral masses	Potter sequence, renal failure in infancy, hypertension, portal HTN
Extrarenal	Hepatic cysts (>50%), Berry aneurysms, Mitral Valve Prolapse	Congenital Hepatic Fibrosis, Biliary Dysgenesis

Renal Dysplasia & Medullary Cysts - Disorganized Development

Multicystic Dysplastic Kidney (MCDK): Sporadic, non-hereditary anomaly where the kidney is replaced by non-communicating cysts.
- Histology: Cysts are lined by flattened epithelium, surrounded by undifferentiated mesenchyme and islands of cartilage.
Medullary Sponge Kidney (MSK): Cystic dilation of the medullary collecting ducts.
- Presents with recurrent kidney stones and hematuria; associated with nephrocalcinosis.

⭐ Exam Favorite: Unilateral MCDK is a common cause of an abdominal mass in newborns. Bilateral involvement leads to Potter sequence and is incompatible with life.

High‑Yield Points - ⚡ Biggest Takeaways

Potter sequence results from oligohydramnios (e.g., bilateral renal agenesis), causing pulmonary hypoplasia and limb/facial defects.

A horseshoe kidney's ascent is blocked by the inferior mesenteric artery (IMA); it is associated with Turner syndrome.

Multicystic dysplastic kidney is a unilateral, non-functional kidney composed of cysts and abnormal tissue.

ARPKD presents in infants and is associated with hepatic fibrosis.

ADPKD presents in adults and is associated with berry aneurysms and liver cysts.

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