Right parasternal long-axis four-chamber view of a Cavalier King Charles Spaniel (CKCS) affected by myxomatous mitral valve disease (MMVD) in American College of Veterinary Internal Medicine (ACVIM) class D; severe mitral valve prolapse was assessed by the protrusion of one or both leaflets over line T. The arrows outline the severity of mitral valve prolapse and the affected leaflet. Line P is drawn from the hinge point of the anterior leaflet to the hinge point of the posterior leaflet. Line T is drawn from the middle of the elliptical echogenic area at the lower part of the atrial septum to the atrioventricular junction, i.e., the junction between the left ventricular wall/annulus fibrosis and the left atrial wall.