A Food and Drug Administration advisory committee has called on the agency to approve a brain scan that verifies Alzheimer’s disease in a patient. The product works by identifying beta amyloid plaques, a hallmark sign of the condition. If the scanner goes to market, the result could be much earlier confirmation of a disease that now can be verified only with an autopsy. Supporters said the product might also lead to new treatments for the disease, which affects more than 4 million people nationwide.