**Core Concept**
The question is testing the clinical correlation between the patient's physical characteristics and the cardiac murmur, specifically the association between tall stature, arm span, and aortic root dilatation.

**Why the Correct Answer is Right**
The patient's arm span is significantly greater than their height, which is a characteristic feature of Marfan syndrome, a genetic disorder that affects the body's connective tissue. This condition often leads to aortic root dilatation, which can cause aortic regurgitation (AR), a condition characterized by the leakage of the aortic valve, resulting in a diastolic murmur best heard in the second right intercostal space. The increased arm span and height are due to the elongation of the long bones and the aorta, which is a hallmark of Marfan syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not correlate with the patient's physical characteristics and the location of the murmur. Aortic stenosis is typically associated with a systolic murmur, not a diastolic murmur.
**Option B:** This option is incorrect because mitral regurgitation is not typically associated with a diastolic murmur, and it is best heard at the apex, not the second right intercostal space.
**Option C:** This option is incorrect because hypertrophic cardiomyopathy (HCM) typically presents with a systolic murmur, not a diastolic murmur, and it is not directly related to the patient's physical characteristics.

**Clinical Pearl / High-Yield Fact**
In patients with Marfan syndrome, aortic root dilatation is a significant concern, and regular echocardiographic monitoring is essential to prevent aortic dissection or rupture.

**Correct Answer:** B. Mitral regurgitation