Rheumatic heart disease is a major complication of rheumatic fever, triggered by group A streptococcal throat infections, leading to inflammation of the heart and potential valve damage. It primarily affects children in developing nations and is characterized by symptoms such as carditis, polyarthritis, and chorea, with significant laboratory findings including elevated inflammatory markers and echocardiographic evidence of heart involvement. Long-term effects include severe valvular diseases, particularly affecting the mitral valve, which can result in heart failure and other serious complications.