I registered just so I could reply to this thread, because I was thinking about just this topic tonight, stumbled across this board, and it struck me that it's actually pretty easily explained just what is meant by Crusher "heading" up Starfleet Medical. I can think of a few analogous situations in the Trek canon off the top of my head: recall, for instance, how a young but ambitious Lt. Commander Shelby was "heading up" Starfleet's Borg business prior to the first Borg incursion because of her apparent expertise on the Borg--Admiral Hanson makes note to Picard of how she "took charge" and "put them on track" or something along those lines. After the first Borg threat was eventually squashed, Shelby was then given some kind of important position overseeing the rebuilding of the fleet, similarly because of her experience with the anti-borg defenses and weapons she and her team had been devising--she may have even been responsible for including Sisko in those efforts, himself probably still a relatively young Lt. Commander at the time (as he was on the Saratoga at Wolf 359), who helped design the Defiant prototype. Years later, Sisko convinces Starfleet Brass to make Lt. Commander Worf the Chief of Strategic Operations for the sector (a sector that was strategically vital to the security of the Federation) during the Klingon conflict, because of Worf's expertise with Klingon culture and military practices. And Captain Sisko, because of his experience with the Dominion, was made Acting Head of Starfleet Security on Earth by order of Admiral Leyton (albeit in Leyton's unlawful attempt to place Earth and the Federation under martial law), not to mention Admiral Ross and Starfleet delegating much of the strategic planning to him during the Dominion War. Similarly, Picard was given authority over the entire task force assigned to negating the Romulan involvement in the Klingon Civil war, presumably because of his experience with the cat and mouse games the Romulans had played with the Enterprise since first re-appearing at the end of season one. Now, based on these examples, we can probably assume that Dr. Crusher was asked to "head up" Starfleet Medical because of some particular expertise of hers that was relevant to a pressing issue that Starfleet Medical was facing at the time, perhaps a threat that she herself had encountered in her first year on the Enterprise (the Tarellian Plague for instance, or the Tsiolkovsky Virus). Whatever it was, it doesn't seem to be unheard of in the Trek universe that a lower-ranking officer would be, at least temporarily, put in charge of high-level operations!