A 38-year-old female presents to the clinic complaining of severe, generalized muscle weakness that has progressively worsened over the past three days. She has a past medical history of Sjögren syndrome, for which she regularly uses artificial tears and saliva substitutes. She denies diarrhea, vomiting, or recent changes to her diet. Vital signs are within normal limits. Laboratory studies are obtained and reveal the following:
| Laboratory Test | Result | Reference Range |
|---|---|---|
| Sodium | 138 mEq/L | 135-145 mEq/L |
| Potassium | 2.1 mEq/L | 3.5-5.0 mEq/L |
| Chloride | 114 mEq/L | 98-106 mEq/L |
| Bicarbonate | 14 mEq/L | 22-28 mEq/L |
| Blood urea nitrogen | 12 mg/dL | 8-20 mg/dL |
| Creatinine | 0.8 mg/dL | 0.6-1.2 mg/dL |
Urine studies show:
| Laboratory Test | Result |
|---|---|
| pH | 6.8 |
| Sodium | 40 mEq/L |
| Potassium | 35 mEq/L |
| Chloride | 30 mEq/L |
Which of the following mechanisms best explains this patient's laboratory findings?
- Impaired distal tubular secretion of hydrogen ionsAnswer
- BDecreased proximal tubular reabsorption of bicarbonate
- CReduced collecting duct responsiveness to aldosterone
- DExcessive gastrointestinal loss of bicarbonate-rich fluids
- EIncreased distal delivery of sodium and water due to diuretic use
Answer
Impaired distal tubular secretion of hydrogen ions
The correct answer accurately describes the pathophysiology of Type 1 (distal) renal tubular acidosis (RTA). This patient presents with a non-anion gap metabolic acidosis (serum anion gap = 10 mEq/L) and severe hypokalemia. To determine the etiology, the urine anion gap (UAG) must be calculated: . A positive UAG indicates low urinary ammonium excretion, pointing to a renal acidification defect rather than gastrointestinal bicarbonate loss. The combination of hypokalemia and an inappropriately alkaline urine pH (6.8) in the setting of acidemia confirms an inability of the intercalated cells in the distal tubule to secrete hydrogen ions. Sjögren syndrome is a classic autoimmune condition associated with the development of Type 1 RTA.
Step-by-Step Solution
Key Concept
Diagnosis of Type 1 (distal) renal tubular acidosis via interpretation of non-anion gap metabolic acidosis, hypokalemia, positive urine anion gap, and inappropriately high urine pH.