What is the most appropriate diagnostic step for determining the cause of a chronic cough in a young dog?

Tracheobronchoscopy is the most appropriate diagnostic step for determining the cause of a chronic cough in a young dog because it allows for direct visualization of the trachea and bronchi. This procedure enables the veterinarian to identify any abnormalities such as foreign bodies, tumors, inflammation, or infections in the airways that could be contributing to the cough. Additionally, samples can often be collected for cytology or cultures during the procedure, which can provide further insights into infectious causes or specific pathologies.

In contrast, thoracic radiographs, while useful in evaluating the overall structure of the lungs and ruling out significant pathologies such as tumors or serious infections, do not provide the same level of detail about the airways themselves. Blood work for infectious disease can help identify systemic infections but may not directly pinpoint the cause of a localized respiratory issue. Fecal examination is generally unrelated to respiratory conditions and would not be relevant in the workup of a chronic cough.