A 35-year-old woman presents to the clinic with a one-month history of progressive muscle weakness and fatigue. She also reports a gritty sensation in her eyes and feeling the need to drink water frequently to comfortably swallow dry foods. Vital signs are within normal limits. Physical examination is notable for dry oral mucosa and 4/5 muscle strength in the proximal lower extremities.
Laboratory results are as follows:
Serum chemistry
| Analyte | Value | Reference Range |
|---|---|---|
| Sodium | 139 mEq/L | 136-145 mEq/L |
| Potassium | 2.5 mEq/L | 3.5-5.0 mEq/L |
| Chloride | 114 mEq/L | 98-106 mEq/L |
| Bicarbonate | 15 mEq/L | 22-28 mEq/L |
| Blood urea nitrogen | 12 mg/dL | 7-18 mg/dL |
| Creatinine | 0.8 mg/dL | 0.6-1.2 mg/dL |
Arterial blood gas (room air)
| Parameter | Value | Reference Range |
|---|---|---|
| pH | 7.28 | 7.35-7.45 |
| pCO2 | 30 mm Hg | 35-45 mm Hg |
| pO2 | 95 mm Hg | 75-105 mm Hg |
Urine studies
| Parameter | Value | Reference Range |
|---|---|---|
| pH | 6.5 | 4.5-8.0 |
| Sodium | 65 mEq/L | Variable |
| Potassium | 35 mEq/L | Variable |
| Chloride | 40 mEq/L | Variable |
Based on the provided laboratory data, which of the following is the most likely underlying mechanism for this patient's electrolyte and acid-base disturbances?
- Impaired hydrogen ion secretion in the distal tubuleAnswer
- BDefective bicarbonate reabsorption in the proximal tubule
- CAldosterone resistance at the cortical collecting duct
- DExtrarenal bicarbonate loss with appropriate renal compensation
- ERenal chloride loss with secondary hyperaldosteronism
Answer
Impaired hydrogen ion secretion in the distal tubule
This patient is presenting with a normal anion gap metabolic acidosis, indicated by a serum anion gap of (). To differentiate between extrarenal bicarbonate loss (e.g., diarrhea) and impaired renal acid excretion (renal tubular acidosis), the urine anion gap (UAG) should be calculated. The UAG in this patient is (). A positive UAG indicates that the kidneys are not excreting sufficient ammonium (), confirming a renal etiology. Among the RTAs, the presence of hypokalemia points to either Type 1 or Type 2, while the inappropriately elevated urine pH (6.5) despite severe systemic acidemia is the hallmark of Type 1 (distal) RTA. Type 1 RTA is characterized by impaired hydrogen ion secretion in the alpha-intercalated cells of the distal tubule and is frequently associated with autoimmune disorders, such as Sjögren syndrome (strongly suggested by this patient's dry eyes and dry mouth).
Step-by-Step Solution
Key Concept
Normal anion gap metabolic acidosis with a positive urine anion gap, hypokalemia, and urine pH > 5.5 is diagnostic of Type 1 (distal) renal tubular acidosis, which is commonly associated with autoimmune conditions like Sjögren syndrome.