Answer: A

Reasoning: Unequal blood pressures in the two arms point to disease of the proximal aorta or its great branches, and an early diastolic murmur signifies significant aortic regurgitation (AR). AR commonly accompanies:
• Takayasu arteritis, where aortitis dilates the root and stenoses branch vessels, giving pulse/BP discrepancies [Hashimoto 1996, PMID 9119520].  
• Proximal (pre-subclavian) coarctation, often paired with a bicuspid valve that leaks [Poliakova 1996, PMID 9324790].  
• Type-A aortic dissection, which both damages the valve and occludes arch branches, producing arm BP differences [Gustavsson 1988, PMID 3354351].

Supravalvular aortic stenosis, however, characteristically produces an ejection systolic murmur without significant AR; reports of AR are only rare case-reports, not the rule [Mohan 2011, PMID 21976893]. Hence it is the least likely explanation for the combination described.