LOUISVILLE, Ky. &ndash; To miss the Kentucky Derby is disappointing. To not know why is disconcerting.\r\nIn announcing Friday morning that morning-line second choice Uncle Mo would be scratched from Saturday&rsquo;s Kentucky Derby, owner Mike Repole and trainer Todd Pletcher could not pinpoint the exact nature of Uncle Mo&rsquo;s problem. Three veterinarians have examined Uncle Mo, but other than a one elevated enzyme, there is no definitive diagnosis.\r\n&ldquo;He&rsquo;s got one specific enzyme that&rsquo;s elevated that has everyone baffled; they can&rsquo;t identify why,&rdquo; Pletcher said at a Friday morning press conference in the auxiliary press box at Churchill Downs. &ldquo;Generally, without getting too scientific, when this particular enzyme is elevated there is also something in the blood work that would lead them to the direction toward a liver or a kidney [issue], but this specific case it&rsquo;s one single enzyme that&rsquo;s elevated that doesn&rsquo;t really lead you anywhere else.&rdquo;\r\nKENTUCKY DERBY DAY: Past performances, contender profiles, and video\r\nThe only thing for sure is that the problem is not soundness related. Uncle Mo did have a strong 1 1/2-mile gallop shortly after 6 a.m. Friday morning, but he did not come off the track with a high energy level that has been his trademark.\r\nUncle Mo was diagnosed with a gastrointestinal tract infection following his third-place finish in the Wood Memorial on April 9. Uncle Mo has been on a variety of medications to treat the infection. As the Derby drew closer, Pletcher had to take Uncle Mo off of those medications and when he did Pletcher said &ldquo;we saw a little deterioration the last couple of days.&rdquo;\r\n&ldquo;The biggest thing I see is depressed appetite, loss of weight, and his hair coat&rsquo;s not right,&rdquo; Pletcher said. &ldquo;When he was in the Breeders&rsquo; Cup last year, he looked phenomenal; immense hair coat, dapples, rich color, good appetite, good weight, good muscle tone all the things you look for in any horse&rsquo;s overall health. Unfortunately, he doesn&rsquo;t have those things, but we can&rsquo;t find why. &hellip; I don&rsquo;t know what it is, and nobody else knows what it is.&rdquo;\r\nUntil it can be determined what exactly is ailing Uncle Mo, his racing future is uncertain. In order to facilitate a quicker diagnosis, it is likely that Uncle Mo would be shipped to an equine clinic in Lexington, Ky., in the near future.\r\n&ldquo;Most likely, we&rsquo;ll go somewhere,&rdquo; Pletcher said. &ldquo;That&rsquo;s something that Mike and I will talk about later.&rdquo;\r\nSaid Repole: &ldquo;We&rsquo;re not worried about his race schedule, we&rsquo;re worried about the horse.&rdquo;\r\nThough the announcement wasn&rsquo;t made until Friday morning, Repole said he made the decision to scratch Thursday morning after Pletcher told him he didn&rsquo;t want to run the horse. That came after Repole and Pletcher conferred with three veterinarians &ndash; Dr. Ken Reid, Dr. Doug Byars, and Dr. Steve Allday &ndash; who couldn&rsquo;t come up with a firm diagnosis. Repole said out of respect to the three veterinarians who examined Uncle Mo, he wanted to wait until they performed further tests to announce the scratch.\r\n&ldquo;The decision was an easy decision because Todd Pletcher said he was not running,&rdquo; Repole said.\r\nCoincidentally, Pletcher found himself in a similar situation last year when he had to declare probable Derby favorite Eskendereya from the Derby six days before the race. He won the race with Super Saver. This year, despite losing Uncle Mo, Pletcher and Repole still had Stay Thirsty to run.\r\n&ldquo;Its&rsquo; very, very, very, very, very disappointing,&rdquo; Pletcher said. &ldquo;I said last year honestly I don&rsquo;t think I&rsquo;ve ever had a horse as good as Uncle Mo. To not make it here is a big letdown. I take it as a personal failure. I feel extremely bad for Mike. He&rsquo;s given us every resource possible to try and get this thing corrected. We ran out of time.\r\n&ldquo;It&rsquo;s very, very frustrating but what we&rsquo;re really concerned about is the horse&rsquo;s health,&rdquo; Pletcher added. &ldquo;We got something going on inside. I don&rsquo;t know what it is, the best vets in the world don&rsquo;t know what it is. Sometimes you can deal with issues when you know what they are you know what&rsquo;s okay and what&rsquo;s not okay. But when you don&rsquo;t know, that&rsquo;s when you get scared.&rdquo;\r\nThe decision to scratch Uncle Mo was difficult for Repole because he firmly believes he has the best 3-year-old in the country. Uncle Mo was certainly the best 2-year-old last year, winning a maiden race by 14 1/4 lengths at Saratoga in August, the Champagne by 4 3/4 lengths in a time that equaled Seattle Slew for the second-fastest one-mile Champagne ever run, and a tour-de-force 4 1/4-length victory in the Breeders&rsquo; Cup Juvenile here at Churchill. He was voted the Eclipse Award as North America&rsquo;s best 2-year-old.\r\n&ldquo;Does it hurt that he&rsquo;s not here?&rdquo; Repole said. &ldquo;Yes, because Uncle Mo, in my opinion, he&rsquo;s five to seven lengths better than any horse in this race.&rdquo;\r\nSaid Pletcher: &ldquo;I told Mike this decision is really, really going to stink on May 7, but let&rsquo;s hope the rest of the year when they run the Jim Dandy, Travers, and Breeders&rsquo; Cup Classic, and those type of races it&rsquo;s going to be real good.&rdquo;\r\nIt marked the third straight year that jockey John Velazquez lost a top contender for the Derby. In 2009, Quality Road was declared the Monday before the Derby due to quarter crack. In 2010, Velazquez was the rider of Eskendereya, who suffered a career-ending leg injury. Velazquez did pick up the mount on Animal Kingdom, whose assigned rider, Robby Albarado, suffered a broken nose in a post parade accident Thursday.