Anxiety or Panic Disorder: Dyspnea on exertion can be a symptom of anxiety or panic disorder, and a negative stress test would rule out cardiac causes, making this a plausible explanation. The physical symptoms of anxiety can mimic those of cardiac issues, leading to confusion in diagnosis.

Asthma or Chronic Obstructive Pulmonary Disease (COPD): Both conditions can cause dyspnea on exertion. Although a stress test is negative for cardiac issues, pulmonary function tests would be needed to diagnose these conditions.
Obesity: Excess weight can lead to dyspnea on exertion due to the increased demand on the body during physical activity. A negative stress test does not rule out the effects of obesity on respiratory and cardiovascular systems during exertion.
Deconditioning: Lack of physical fitness can cause dyspnea on exertion. A negative stress test indicates no cardiac issue, but deconditioning affects the body's ability to efficiently use oxygen and can lead to shortness of breath during exertion.

Pulmonary Embolism: Although less likely, a pulmonary embolism can cause dyspnea on exertion and may not always be detected by a stress test, which is primarily designed to evaluate cardiac function. This condition is potentially life-threatening and requires immediate medical attention.
Cardiac Sarcoidosis or Other Infiltrative Cardiomyopathies: These conditions can lead to heart failure and dyspnea on exertion but might not be detected by a standard stress test. They are less common but critical to diagnose due to their potential impact on heart function.
Hypothyroidism: Severe, untreated hypothyroidism can lead to dyspnea on exertion among other symptoms. While not immediately life-threatening like a pulmonary embolism, it is crucial to diagnose and treat to prevent long-term health issues.

Pulmonary Arterial Hypertension (PAH): A rare but serious condition that can cause dyspnea on exertion. Diagnosis typically requires right heart catheterization and would not be detected by a standard stress test.
Mitral Valve Prolapse with Severe Regurgitation: While often asymptomatic, in severe cases, it can lead to dyspnea on exertion. A negative stress test does not rule out valvular heart disease, which would require echocardiography for diagnosis.
Interstitial Lung Disease: Conditions like idiopathic pulmonary fibrosis can cause dyspnea on exertion. Diagnosis involves imaging studies like high-resolution CT scans and pulmonary function tests, not a stress test.