Which of the following is the most likely cause for the findings demonstrated in this...

The video shows thickened, retracted, and possibly regurgitant valve leaflets with a characteristic appearance seen in drug-induced valvulopathy. Drugs such as ergot derivatives (e.g., methysergide) and appetite suppressants (e.g., fen-phen) can cause fibrotic thickening of valve leaflets mimicking carcinoid heart disease or rheumatic valve disease.

Infective endocarditis presents with vegetations and potentially valve destruction but typically not the diffuse thickening seen here. Rheumatic fever causes leaflet thickening but has a different chronic clinical course. Systemic lupus may cause valve thickening but often involves Libman-Sacks vegetations rather than diffuse fibrosis.

This is discussed in the "Textbook of Clinical Echocardiography, 6e", Chapter on Valvular Heart Disease - Drug Induced and Secondary Causes【20:400-405†Textbook of Clinical Echocardiography】.