What is a characteristic finding in an obstructive lung disease when evaluated using spirometry?

In obstructive lung diseases, such as asthma or chronic obstructive pulmonary disease (COPD), the primary characteristic observed on spirometry is a decrease in the forced expiratory volume in one second (FEV1) relative to the forced vital capacity (FVC). This leads to a lower FEV1/FVC ratio, indicating airflow restriction.

The correct answer highlights that FEV1 is decreased, which reflects the patient's difficulty in expiring air quickly due to narrowed airways. However, the FVC may be normal or decreased. In many patients with obstructive lung disease, the total lung capacity may be maintained or even elevated due to hyperinflation, but they can still expel less air in the FEV1 measurement. This differentiates obstructive lung diseases from restrictive lung diseases, where both FEV1 and FVC would usually be decreased, but the ratio would remain relatively normal or increase.

Thus, the emphasis on the normal or decreased FVC aligns with the understanding that while airflow obstruction is present, lung volumes may not be as restricted as seen in conditions that affect lung expansion.