Review Question - QID 214947

A 33-year-old woman presents to the emergency department with muscle spasms. She has noticed "twitching" in both her upper and lower extremities over the past 3 days that have interfered with her training for a marathon. She has felt more fatigued over the past month. She has no significant medical history. She takes no medication other than whey protein supplements, a multivitamin, and fish oil. Her temperature is 98.5°F (36.9°C), blood pressure is 103/71 mmHg, pulse is 105/min, respirations are 11/min, and oxygen saturation is 100% on room air. Physical exam reveals a thin woman with thinning hair. She appears pale and her mucous membranes are dry. Her neurological and cardiac exams are unremarkable. Laboratory studies are ordered as seen below.

Hemoglobin: 10 g/dL
Hematocrit: 30%
Leukocyte count: 6,100/mm^3 with normal differential
Platelet count: 187,500/mm^3

Serum:
Na+: 130 mEq/L
Cl-: 100 mEq/L
K+: 2.3 mEq/L
HCO3-: 28 mEq/L
BUN: 40 mg/dL
Glucose: 79 mg/dL
Creatinine: 0.9 mg/dL
Ca2+: 8.2 mg/dL
Mg2+: 1.8 mg/dL
Thyroid stimulating hormone (TSH): 4.0 mIU/L
Free T4: 0.4 ng/dL (normal: 0.7-1.53 ng/dL)

The patient is given 4 liters of lactated ringer solution and 40 mEq of potassium. Her repeat whole blood potassium 3 hours later is 2.4 mEq/L, and her repeat BUN is 20 mEq/L. Another 40 mEq of potassium is administered. Another repeat whole blood potassium is 2.5 mEq/L. The patient’s muscle spasms persist. Which of the following is the most likely underlying etiology for this patient’s chief concern?