Review Question - QID 104015

A 70-year-old female with a history of congestive heart failure presents to the emergency room with dyspnea. She reports progressive difficulty breathing which began when she ran out of her furosemide and lisinopril prescriptions 1-2 weeks ago. She states the dyspnea is worse at night and when lying down. She denies any fever, cough, or GI symptoms. Her medication list reveals she is also taking digoxin. Physical exam is significant for normal vital signs, crackles at both lung bases and 2+ pitting edema of both legs. The resident orders the medical student to place the head of the patient's bed at 30 degrees. Additionally, he writes orders for the patient to be given furosemide, morphine, nitrates, and oxygen. Which of the following should be checked before starting this medication regimen?