Review Question - QID 109464

A 46-year-old woman presents to her primary care doctor to request a referral to ophthalmology. Her vision has been steadily declining over the past 2 years and she thinks she needs a prescription for glasses. She further complains that her lips and feet feel numb. Her medical history is notable for medullary thyroid cancer status post total thyroidectomy. Her periods are regular. She enjoys a diverse diet and takes levothyroxine. Her blood pressure is 110/70 mmHg, pulse is 80/min, and respirations are 12/min. She is alert and oriented. Her pupils are equal, round, and reactive to light, but appear opacified. Extraocular movements are intact, albeit slow. Her visual acuity is decreased bilaterally. The remainder of her physical exam is unremarkable. Her basic metabolic panel is shown below:

Na+: 139 mEq/L
Cl-: 100 mEq/L
K+: 4.9 mEq/L
HCO3-: 25 mEq/L
BUN: 10 mg/dL
Glucose: 110 mg/dL
Creatinine: 0.8 mg/dL
Thyroid-stimulating hormone: 1.5 µU/mL
Ca2+: 7.0 mEq/L
Phosphorus: 6.5 mEq/L
Albumin: 3.6 mg/dL

The patient's ECG is shown in Figure A. What is the most likely cause of this presentation?