You are answering a USMLE-style medical question. Question: A 72-year-old woman is admitted to the intensive care unit for shortness of breath and palpitations. A cardiac catheterization is performed and measurements of the left ventricular volume and pressure at different points in the cardiac cycle are obtained. The patient's pressure-volume loop (gray) is shown with a normal pressure-volume loop (black) for comparison. Which of the following is the most likely underlying cause of this patient's symptoms? Options: A) Mitral valve regurgitation B) Increased systemic vascular resistance C) Increased ventricular wall stiffness D) Impaired left ventricular contractility Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.