Review Question - QID 210895

A 38-year-old woman presents to her primary care physician for a new patient appointment. She states that she feels well and has no current complaints. The patient recently started seeing a specialist for treatment for another medical condition but otherwise has had no medical problems. She lives alone and drinks 2 alcoholic beverages every night. She has had 3 sexual partners in her lifetime, uses oral contraceptive pills for contraception, and has never been pregnant. Her temperature is 98.7°F (37°C), blood pressure is 114/68 mmHg, pulse is 90/min, respirations are 12/min, and oxygen saturation is 99% on room air. Physical exam reveals a pleasant, obese woman with a normal S1 and S2 on cardiac exam. A musculoskeletal exam reveals swelling of the MCP and PIP joints of the hands as well as ulnar deviation of the fingers. Laboratory tests are below:

Serum:
Na+: 139 mEq/L
Cl-: 100 mEq/L
K+: 4.3 mEq/L
HCO3-: 25 mEq/L
BUN: 20 mg/dL
Glucose: 99 mg/dL
Creatinine: 1.1 mg/dL
Ca2+: 10.2 mg/dL
AST: 95 U/L
ALT: 68 U/L

Her previous laboratory tests 2 months ago were unremarkable. Which of the following is the most appropriate explanation for this patient's abnormal laboratory values?