A 10-year-old pit bull presents with exercise intolerance and pulmonary crackles. What is the most likely diagnosis?

In a 10-year-old pit bull presenting with exercise intolerance and pulmonary crackles, the most likely diagnosis is idiopathic pulmonary fibrosis. This condition is characterized by progressive scarring (fibrosis) of the lung tissue, which leads to decreased lung function. In older dogs, idiopathic pulmonary fibrosis can manifest with respiratory signs, including exercise intolerance due to reduced oxygen exchange, as well as crackling sounds upon auscultation, which is indicative of abnormal lung architecture and inflammation.

Chronic bronchitis, while a potential consideration, typically presents with a chronic cough and might not initially cause pronounced exercise intolerance and crackles. Instead, chronic bronchitis is more often associated with a history of recurrent cough and increased respiratory effort over time, rather than the abrupt presentation often seen in age-related pulmonary fibrosis.

Heartworm disease could also present with exercise intolerance and pulmonary signs due to associated respiratory complications. However, in a dog of this age, idiopathic pulmonary fibrosis is a more common diagnosis, particularly in the absence of other symptoms that might strongly suggest an active heartworm infection, such as weight loss or significant coughing.

Pneumonia, particularly the acute form, would usually present with more systemic signs like fever, lethargy, and potentially productive cough, which