Asthma: A positive bronchodilator response is highly suggestive of asthma, which is characterized by reversible airflow obstruction. The presence of mild obstructive airflow on spirometry and a normal DLCO (diffusing capacity of the lungs for carbon monoxide) further supports this diagnosis, as asthma typically does not affect gas exchange significantly.

Chronic Obstructive Pulmonary Disease (COPD) - Mild: Although COPD is typically associated with a less significant bronchodilator response compared to asthma, mild COPD can sometimes exhibit reversibility. The normal DLCO and mild obstruction could fit a diagnosis of early or mild COPD.
Bronchiectasis: This condition can cause obstructive symptoms and spirometric findings similar to asthma or COPD. However, the presence of a positive bronchodilator response and normal DLCO might be less typical, making it somewhat less likely but still a consideration.

Cystic Fibrosis: Although less common, cystic fibrosis can present with obstructive lung disease and might show some reversibility with bronchodilators. It's crucial to consider this diagnosis due to its significant implications for management and prognosis.
Alpha-1 Antitrypsin Deficiency: This genetic disorder can lead to COPD-like symptoms at a younger age and might show some response to bronchodilators. Given its potential for severe lung disease and the importance of early diagnosis for family screening and potential liver disease, it should not be missed.

Eosinophilic Granulomatosis with Polyangiitis (Churg-Strauss Syndrome): A rare autoimmune condition that can cause asthma-like symptoms and obstructive lung disease. The diagnosis is often suggested by the presence of eosinophilia and other systemic symptoms.
Obstructive Sleep Apnea with Associated Asthma-like Symptoms: Although not typically diagnosed through spirometry, obstructive sleep apnea can sometimes present with symptoms overlapping with asthma, such as wheezing or cough, particularly if there's associated obesity or other complicating factors.