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Difficulty: HardNephrology

A 38-year-old woman is evaluated in the emergency department for severe, progressive muscle weakness over the past two days. She reports feeling so weak today that she had difficulty standing up from a chair. Her medical history is significant for a 2-year history of dry eyes and dry mouth, for which she uses artificial tears and saliva substitutes. She takes no other medications. Her temperature is 37.0°C (98.6°F), blood pressure is 110/70 mmHg, pulse is 88/min, and respirations are 18/min. Physical examination reveals dry oral mucosa and 4/5 motor strength in the proximal muscles of the upper and lower extremities. Sensation and deep tendon reflexes are intact.

Laboratory studies show the following:
TestResult
Serum sodium140140 mEq/L
Serum potassium2.42.4 mEq/L
Serum chloride115115 mEq/L
Serum bicarbonate1313 mEq/L
Blood urea nitrogen1212 mg/dL
Serum creatinine0.80.8 mg/dL
Urine studies:
TestResult
pH6.86.8
Sodium5050 mEq/L
Potassium2020 mEq/L
Chloride3030 mEq/L

Based on these findings, which of the following is most likely to be found on further evaluation of this patient?

  1. Bilateral nephrocalcinosis on renal ultrasoundAnswer
  2. B
    Uric acid crystals on urine microscopy
  3. C
    Impaired proximal tubule reabsorption of amino acids
  4. D
    A negative calculated urine anion gap
  5. E
    Elevated arterial carbon dioxide tension on blood gas analysis

Answer

Bilateral nephrocalcinosis on renal ultrasound
This patient's laboratory profile reveals a normal anion gap metabolic acidosis (NAGMA) with severe hypokalemia. The calculated urine anion gap is positive (+40+40), which indicates a failure of the kidneys to excrete ammonium (NH4+NH_4^+), ruling out gastrointestinal bicarbonate loss. The urine pH is 6.8, which is inappropriately alkaline in the setting of severe systemic acidemia. This inability to acidify the distal tubule is the hallmark of Type 1 (distal) Renal Tubular Acidosis (RTA), which is strongly associated with autoimmune disorders like Sjögren syndrome (suggested by her dry eyes and mouth). Chronic acidemia buffers against the bone, releasing calcium (hypercalciuria), while the proximal tubule reabsorbs more citrate (hypocitraturia). The triad of hypercalciuria, hypocitraturia, and an alkaline urine pH highly predisposes these patients to the precipitation of calcium phosphate stones, resulting in bilateral nephrocalcinosis on imaging.

Step-by-Step Solution

1
Calculate the serum anion gap (AG).
AG=140−(115+13)=12AG = 140 - (115 + 13) = 12 mEq/L.
Determines if the metabolic acidosis is anion-gap or normal anion-gap. A result of 12 indicates a Normal Anion Gap Metabolic Acidosis (NAGMA).
2
Evaluate the serum potassium level.
Severe hypokalemia is present (K+=2.4K^+ = 2.4 mEq/L).
Narrows the differential diagnosis of NAGMA to gastrointestinal losses (like diarrhea) or Type 1 / Type 2 Renal Tubular Acidosis (RTA).
3
Calculate the Urine Anion Gap (UAG).
UAG=(Urine Na++Urine K+)−Urine Cl−=(50+20)−30=+40UAG = (Urine\ Na^+ + Urine\ K^+) - Urine\ Cl^- = (50 + 20) - 30 = +40.
Differentiates GI vs. renal bicarbonate loss. A positive UAG indicates impaired renal ammonium (NH4+NH_4^+) excretion, confirming a renal etiology (RTA) rather than diarrhea.
4
Assess the urine pH to differentiate the type of RTA.
Urine pH is 6.86.8, which is inappropriately alkaline (>5.5> 5.5) for severe systemic acidemia.
An inability to acidify the urine below a pH of 5.5 confirms a distal H+H^+ secretion defect, diagnostic of Type 1 (distal) RTA.
5
Identify downstream complications of Type 1 RTA.
Alkaline urine + hypercalciuria (from bone buffering) + hypocitraturia -> Calcium phosphate stone formation.
Predicts the clinical sequelae that would be found on imaging (nephrocalcinosis).

Key Concept

Diagnosis and complications of Type 1 (Distal) Renal Tubular Acidosis (RTA)
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Topics

Nephrology

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