Q: What is MacCallum's patch? 


 Answer: During mitral regurgitation, where the regurgitant jet strikes back the atrial wall, results in endocardial thickening and it is called MacCallum's patch. They are also called MacCallum plaques and also considered as a hallmark in rheumatic heart disease. They are described as "map-like areas of thickened, roughened, and wrinkled part of the endocardium in the left atrium", usually associated with dilated left atrium.

Clinical significance: They are more prone to have vegetations in infective endocarditis.