A 45-year-old woman is brought to the emergency department due to progressive muscle weakness. Over the past week, she has had difficulty rising from a chair and climbing stairs. She has a 5-year history of primary Sjögren syndrome, for which she uses artificial tears and saliva substitutes. Vital signs are within normal limits. Physical examination shows symmetric 3/5 muscle strength in the proximal upper and lower extremities, with decreased deep tendon reflexes.
Laboratory studies are obtained:
| Laboratory Test | Result | Reference Range |
|---|---|---|
| Serum sodium | 140 mEq/L | 136-145 mEq/L |
| Serum potassium | 2.1 mEq/L | 3.5-5.0 mEq/L |
| Serum chloride | 114 mEq/L | 95-105 mEq/L |
| Serum bicarbonate | 14 mEq/L | 22-28 mEq/L |
| Arterial pH | 7.28 | 7.35-7.45 |
| Arterial pCO2 | 30 mmHg | 35-45 mmHg |
| Urine pH | 6.8 | 4.5-8.0 |
Which of the following best explains this patient's clinical presentation and laboratory findings?
- Impaired distal tubular secretion of hydrogen ions with appropriate respiratory compensationAnswer
- BImpaired distal tubular secretion of hydrogen ions with a concurrent primary respiratory alkalosis
- CDecreased production of aldosterone with appropriate respiratory compensation
- DAccumulation of endogenous organic acids with appropriate respiratory compensation
- EGastrointestinal loss of bicarbonate-rich fluid with appropriate respiratory compensation
Answer
Impaired distal tubular secretion of hydrogen ions with appropriate respiratory compensation
The correct answer accurately identifies that the patient has a non-anion gap metabolic acidosis (NAGMA) due to impaired distal tubular hydrogen secretion (Type 1 RTA) with appropriate respiratory compensation. The patient's anion gap is normal at . According to Winter's formula, the expected is ; the measured is , indicating full compensation. The combination of NAGMA, severe hypokalemia (causing the progressive muscle weakness), and an inappropriately basic urine pH () is classic for Type 1 (distal) RTA. This condition is highly associated with autoimmune disorders, particularly primary Sjögren syndrome, due to autoantibody-mediated damage to the alpha-intercalated cells in the distal collecting tubule.
Step-by-Step Solution
Key Concept
Type 1 (distal) renal tubular acidosis (RTA) is classically associated with autoimmune diseases, particularly Sjögren syndrome, and presents with a normal anion gap metabolic acidosis, severe hypokalemia, and an inappropriately basic urine pH (> 5.5).