A 56-year-old woman presents to the emergency department with muscle weakness. Her symptoms have progressively worsened over the course of 2 weeks and are most significant in her lower extremities. She also notices increased urinary frequency. Approximately 1 month ago, she was diagnosed with calcium phosphate nephrolithiasis. Medical history is significant for rheumatoid arthritis diagnosed approximately 10 years ago and treated with methotrexate as well as type 2 diabetes mellitus treated with metformin. Her temperature is 98.6°F (37°C), blood pressure is 138/92 mmHg, pulse is 92/min, and respirations are 17/min. On physical exam, there is mild tenderness to palpation of the metacarpophalangeal and proximal interphalangeal joints. There is 4/5 power throughout the lower extremity. Laboratory testing is shown. Serum: Na+: 137 mEq/L Cl-: 106 mEq/L K+: 2.9 mEq/L HCO3-: 18 mEq/L Glucose: 115 mg/dL Creatinine: 1.0 mg/dL Urine pH: 5.6 Which of the following is the most appropriate next step in management?
Administer intravenous sodium bicarbonate
Begin potassium replacement therapy with dextrose
Increase methotrexate dose
Administer intravenous insulin
Administer intravenous sodium bicarbonate
This patient's non-anion gap metabolic acidosis, hypokalemia, alkaline urine, and history of a calcium phosphate kidney stone and rheumatoid arthritis are suggestive of type 1 (distal) renal tubular acidosis. The treatment of choice is alkali replacement (sodium bicarbonate).

Type 1 (distal) renal tubular acidosis results from impaired acid (H+) secretion in the distal nephron. Impaired acid secretion decreases the kidney's ability to acidify the urine, thus increasing the urine pH. Impaired acid secretion also results in potassium wasting resulting in hypokalemia. Acidemia causes the bone to release calcium phosphate which buffers the acid. The alkaline urine along with decreased citrate excretion (caused by acidemia) leads to a favorable environment to form calcium phosphate stones. Therefore, correction of metabolic acidosis is essential in managing this condition.

Palmer et al. review the evidence regarding the treatment of renal tubular acidosis. They discuss how this disease can be divided into types based on the location of the deficit. They recommend using alkali therapy in patients with distal renal tubular acidosis.

Incorrect Answers:
Answer A: Administering hydrochlorothiazide would be a treatment option for type 2 (proximal) renal tubular acidosis. Patients with this condition can have associated hypokalemia but would be expected to have an acidic urine pH (less than 5.5) after significant bicarbonate excretion.

Answer B: Administering intravenous insulin would further decrease the serum potassium and place the patient at risk of developing cardiac arrhythmia. This patient's most important concern is her renal tubular acidosis that is resulting in muscle weakness from hypokalemia. Her polyuria is caused by hypokalemia, as it decreases the kidney's responsiveness to antidiuretic hormone. Her serum glucose is also not significantly elevated to cause osmotic diuresis.

Answer D: The dextrose (in the replacement therapy with potassium) would trigger the pancreas to secrete insulin and would lead to an intracellular shift of potassium. This would not particularly improve extracellular potassium levels. Treatment should be aimed at addressing the root of the patient's symptoms. Potassium citrate can be administered in cases of severe hypokalemia and hypokalemia that persists after correcting the serum bicarbonate level.

Answer E: Increasing the methotrexate dose is not appropriate in this case. The patient's muscle weakness is more suggestive of hypokalemia secondary to type 1 renal tubular acidosis. She does not report symptoms of inflammatory arthritis (joint pain/stiffness that is worse in the morning and improves with use).

Bullet Summary:
Type 1 (distal) renal tubular acidosis is treated with alkali replacement (sodium bicarbonate or sodium citrate).