A 23-Year-Old Woman, Who Complains of Chronic Cough, Shortness of Breath

A 23-year-old woman, who complains of chronic cough, shortness of breath, and chest tightness is diagnosed with asthma. Which of the following is the best clinical indicator of significant pulmonary obstruction in patients with asthma?

A) Arterial hypoxemia
B) Dyspnea
C) Forced expiratory volume in 1 second (FEV₁) /forced vital capacity (FVC) ratio of less than 75%
D) Large anatomic dead space volume
E) Rapid respirations