Rob Stein The Food and Drug Administration will consider approving treatments for rare diseases based on evidence they have a plausible mechanism. Andrew Harnik/AP hide caption The Food and Drug Administration Monday unveiled the details of a new policy designed to make it easier and quicker for patients with very rare diseases to get cutting-edge treatments. The new guidance would enable the agency to approve new treatments for rare diseases based on evidence for a "plausible mechanism" for how the treatment would work. The policy aims to speed the use of state-of-the-art technologies like gene-editing to create treatments tailored to individual patients suffering from diseases that are so rare that it would be difficult if not impossible to conduct a traditional study first. "For decades families heard the same thing: There are not enough patients. The approval will take too long. You just have to wait for the science to catch up with your child, " Health and Human Services Secretary Robert F. Kennedy Jr.…