An elderly patient is brought to a physician's office after he complained of weakness, dizziness, and nausea while on vacation in another state. Questioning revealed that his symptoms came on...

An elderly patient is brought to a physician's office after he complained of weakness, dizziness, and nausea while on vacation in another state. Questioning revealed that his symptoms came on gradually after he went on a hike in very hot weather the previous day. He also disclosed that he is on a loop diuretic for his hypertension.

If this patient is showing signs of hypokalemia, which fluid was analyzed to diagnose the status of his potassium balance?