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A-D, Photographs showing pulmonary thromboendarterectomy specimens and University of California, San Diego, level classification based on level of disease: level I disease indicates obstructive material in the main pulmonary arteries (A), level II disease starts at the lobar branches (B), level III disease starts at the segmental branches (C), and level IV disease is in the subsegmental branches primarily (D). Level 0 means no disease.
