I left this alone for a day or so and I see you got *(crickets)* for an answer to your question. You are on track I would say. And I agree as I have pointed out before although that is merely an opinion.
Others (including Jerry) may believe that it truly is Betamethasone that the horse tested positive for which would lead to his statement that it is nonsense. If that's the belief, then I understand the nonresponse to your post.
But the idea that these horses tested positive for Beta because of actual use of Beta is difficult for me to believe.
Go back to the Oaklawn positives for Charlatan and Gamine. They were for lidocaine. Baffert won with 3 horses that day at Oaklawn (May 2, 2020): Charaltan by 6, Nadal by 3, and Gamine by a neck. Only 2 of the 3 tested positive despite their argument that it was a pain patch worn by Barnes OR other environmental contamination. The Arkansas Racing Commission let him off the hook likely believing the excuses posed about back patches or test barn problems. Baffert alluded to another horse in the Ark Derby field testing positive also as part of the environmental contamination . . . . I would love to know who it was and who the trainer was for that horse. (I bet I could guess the horse and the trainer if this is true although this was never stated or mentioned by anyone else that another horse had tested positive that I can find).
Gamine goes on to run in the September Kentucky Oaks of 2020 and tests positive for Beta when finishing third. The NY Times had to report the positive as Baffert didn't acknowledge it until after it came to light. He accepted the $1500 fine and DQ from the race and purse forfeiture. I have not found an explanation of this positive as they must have found it prudent to take this joke of a punishment and move on.
Between the Gamine positives, Mereneith out at Del Mar tested positive in July of 2020 for Dextrorphan which is a metabolite of Dexamethasone. The groom was using Dayquil and Nyquil claimed their camp saying another case of environmental contamination.
As we know, in the Derby, MS tests positive for Beta. If you go back and watch the Baffert barn presser where he announced the positive himself and if you can read people a little bit and listen to his exact words, you can probably see that his later claim of Skin Ointment doesn't fit either.
Where it gets interesting for me is the original Ark positives. Lidocaine is usually a numbing agent for a corticosteroid administration as best as I can decipher. I would need someone to teach me far more to make any conclusions or further associations about those cases but it is very interesting that lidocaine has such a marked relation to the administration of corticosteroids (like Beta and Dex let's say).
People far smarter than me in the scientific world can surely connect all these dots on a molecular level. But that wiretapped convo between Rhien and Servis sure says a lot about what Servis was using and how the tests would come back if anything. (Likely as Dex they said).
So, I have no idea what caused the positive tests including MS. It very well could be just lidocaine, dexatrorphan, and betamethasone. But I don't believe this to be so. If his horses were this good as some like to routinely argue, why would he need to use these Mickey Mouse meds so routinely on horses that are winning so easily?
Since we are down this path, let me point out the latest superstar trainer to be allegedly dealing with an issue and a positive test from Ky Derby week. I'll link the Indian Charlie newsletter (a good one overall this time) and you can read about the horse Averly Jane who runs at Keeneland tomorrow. Golden Pal just had a nice run at Keeneland from the same barn to repel the bids of others at the top of the stretch.