Deconditioning: Given the patient's age and the fact that pulmonology and cardiology have cleared her, deconditioning is a likely cause of dyspnea on exertion. The drop in oxygen saturation upon standing could be related to decreased physical fitness and muscle mass, common in elderly individuals.

Anemia: A common condition in the elderly, anemia can cause dyspnea on exertion due to decreased oxygen delivery to tissues. The drop in oxygen saturation upon standing could be exacerbated by anemia.
Autonomic dysfunction: Conditions like orthostatic hypotension or postural orthostatic tachycardia syndrome (POTS) can cause symptoms similar to those described, including dyspnea on exertion and drops in oxygen saturation upon standing.
Sleep disorders: Sleep apnea or other sleep disorders can cause hypoxia, leading to dyspnea on exertion, and may not have been fully evaluated by pulmonology.

Pulmonary embolism: Although pulmonology has cleared the patient, pulmonary embolism can be difficult to diagnose and may not always present with typical symptoms. A high index of suspicion is necessary, especially given the drop in oxygen saturation upon standing.
Cardiac tamponade: Cardiology may have missed a cardiac tamponade, which can cause dyspnea on exertion and drops in oxygen saturation due to impaired cardiac filling.
Hypovolemia or dehydration: Severe dehydration or hypovolemia can cause orthostatic hypotension, leading to decreased oxygen delivery and dyspnea on exertion.

Mitral valve prolapse: A rare condition that can cause dyspnea on exertion, especially if there is significant mitral regurgitation.
Pneumonia or other infectious causes: Although less likely given the lack of other symptoms, pneumonia or other infections can cause hypoxia and dyspnea on exertion.
Neuromuscular disorders: Conditions like myasthenia gravis or amyotrophic lateral sclerosis (ALS) can cause respiratory muscle weakness, leading to dyspnea on exertion.