A 44-year-old woman is having symptoms of muscle weakness and fatigue. On examination, her blood pressure is 120/80 mm Hg, pulse 80/min, JVP 4 cm, heart sounds normal, and lungs clear. Her serum potassium level is 2.5 mEq/L, bicarbonate 18 mEq/L, and anion gap is normal. The urine potassium is 40 mEq/L.For the above patient with hypokalemia, select the most likely diagnosis.