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Difficulty: Very hardNephrology

Match each patient's clinical presentation with the primary pathophysiologic mechanism or underlying diagnosis responsible for their electrolyte and acid-base disturbances.

  • A 35-year-old woman with Sjogren syndrome presents with profound hypokalemia, a normal anion gap metabolic acidosis, and recurrent nephrolithiasis. Her urine pH is 6.8.Impaired hydrogen ion secretion by alpha-intercalated cells in the distal tubule.
  • A 50-year-old man with a monoclonal gammopathy presents with hypokalemia, a normal anion gap metabolic acidosis, and osteomalacia. His urine pH is 5.2.Impaired bicarbonate reabsorption in the proximal convoluted tubule.
  • A 68-year-old man with advanced diabetic nephropathy presents with asymptomatic hyperkalemia and a mild normal anion gap metabolic acidosis. His urine pH is 5.1.Decreased aldosterone synthesis or impaired collecting duct responsiveness to aldosterone.
  • A 22-year-old woman presents with muscle cramps, normal blood pressure, hypokalemia, metabolic alkalosis, and severely decreased urinary calcium excretion.Defect in the sodium-chloride cotransporter (NCCT) in the distal convoluted tubule.
  • A 25-year-old woman with poor dentition and enlarged parotid glands presents with hypokalemia, metabolic alkalosis, and a urine chloride level of 8 mEq/L.Volume depletion leading to secondary hyperaldosteronism and obligate renal potassium wasting.

Answer

The correct matches link each distinct clinical presentation to its core pathophysiologic defect. Type 1 RTA matches to impaired distal hydrogen secretion; Type 2 RTA to impaired proximal bicarbonate reabsorption; Type 4 RTA to hypoaldosteronism; Gitelman syndrome to a distal tubule NCCT defect; and surreptitious vomiting to volume depletion with secondary hyperaldosteronism.
Each vignette represents a pathognomonic presentation for a specific nephrology disorder. Matching requires identifying the disease from the clinical clues (e.g., Sjogren for Type 1 RTA, Myeloma for Type 2 RTA, Diabetes for Type 4 RTA, hypocalciuria for Gitelman, and low urine chloride for vomiting) and linking it to its known pathophysiologic mechanism at the level of the nephron.

Step-by-Step Solution

1
Evaluate the acid-base and potassium status for each clinical vignette.
Patients 1, 2, and 3 have a normal anion gap metabolic acidosis (NAGMA). Patients 4 and 5 have metabolic alkalosis with hypokalemia.
Categorizing into acidosis versus alkalosis immediately narrows the differential diagnosis to Renal Tubular Acidosis (RTA) syndromes versus volume depletion/tubulopathies.
2
Differentiate the NAGMA patients (1, 2, and 3) by their serum potassium levels and urine pH.
Patient 3 has hyperkalemia, which is diagnostic of Type 4 RTA. Patients 1 and 2 have hypokalemia. Patient 1 has a urine pH > 5.5 (diagnostic of Type 1 RTA), while Patient 2 has a urine pH < 5.5 (diagnostic of Type 2 RTA).
Potassium is the key differentiator for Type 4 RTA. Urine pH distinguishes Type 1 (distal failure to acidify) from Type 2 (intact distal acidification once threshold is reached).
3
Differentiate the hypokalemic metabolic alkalosis patients (4 and 5) by their urine chloride levels and physical exam findings.
Patient 5 has a low urine chloride and physical signs of bulimia (vomiting). Patient 4 has hypocalciuria and normal blood pressure without vomiting signs, pointing to Gitelman syndrome.
Urine chloride distinguishes volume-depleted states such as vomiting (< 20 mEq/L) from renal potassium wasting syndromes like Gitelman or Bartter syndrome (> 20 mEq/L).

Key Concept

Differentiation of complex acid-base and electrolyte disorders requires integrating serum potassium, urine pH, urine chloride, and specific clinical features to identify the precise tubular or systemic defect.
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Nephrology

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