Review Question - QID 212448

A 31-year-old man presents to his primary care physician with shortness of breath. He states that he had a “cold” 2 weeks ago and since then has had a persistent cough and worsening shortness of breath. He denies fever, chills, chest pain, sore throat, or rhinorrhea. His medical history is significant for seasonal allergies. He uses fluticasone nasal spray. He had his tonsils removed when he was 8 years of age. His mother and maternal grandfather have cirrhosis, and his father has depression and hypertension. The patient endorses that he smokes tobacco socially on the weekends and uses marijuana daily. He drinks 1-2 beers after work with his co-workers most evenings. A chest radiograph shows hyperinflation of the lungs and hyperlucency. Routine labs are drawn, as shown below.

Serum:
Na+: 139 mEq/L
Cl-: 105 mEq/L
K+: 4.0 mEq/L
HCO3-: 26 mEq/L
Urea nitrogen: 15 mg/dL
Glucose: 100 mg/dL
Creatinine: 0.8 mg/dL
Alkaline phosphatase: 98 U/L
Aspartate aminotransferase (AST, GOT): 46 U/L
Alanine aminotransferase (ALT, GPT): 49 U/L

Pulmonary function tests are pending. Which of the following is most likely to confirm the patient’s diagnosis?