A 20-year-old female presents with a 3-month history of fatigue, weight loss, and dyspnea on exertion. Her physical exam reveals a systolic murmur at the left sternal border that increases with standing and decreases with squatting. Which of the following is the most likely diagnosis?
1) Mitral stenosis
2) Mitral regurgitation
3) Aortic stenosis
4) Aortic regurgitation