Which of the following is a complication of untreated mitral stenosis?

When mitral stenosis is untreated, it leads to an increased pressure in the left atrium due to impaired blood flow from the left atrium to the left ventricle. This prolonged pressure elevation causes the left atrium to dilate and enlarge, resulting in right atrial enlargement.

As pressure within the left atrium increases, it can also lead to complications such as atrial fibrillation, which further exacerbates the risk of thromboembolic events due to stagnant blood flow. The left atrium's enlargement primarily affects the right atrium as well due to the shared anatomical and physiological relationships.

Other complications like left ventricular hypertrophy and pulmonary edema are more directly associated with other cardiac conditions. Cor pulmonale, which refers to right-sided heart failure due to chronic lung disease, might be a secondary consequence of prolonged untreated mitral stenosis but is not a direct complication of it. Therefore, right atrial enlargement is the most relevant complication directly attributable to untreated mitral stenosis.