the science of sport podcast did an in-depth feature on this.
and letsrun typed a lot of it out here:
going through the podcast what jumped out at me was that shelby's positive test closely resembled other tests done on precursors - a substance that encourages the production of another substance (in this case nandrolone).
[WA] have begun noticing a new pattern of carbon isotope signatures in these 19-NA [nandrolone] doping cases. She says that since 2018, 31 conclusive Adverse Analytical Findings for 19-NA belong to one of two distinct groups or types. One batch has an isotope signature around -29‰, while the other is clustered around -23‰. Presumably, the -29‰ is injected nandrolone, but the -23‰ belongs to what Ayotte describes as oral precursors of nandrolone. She even names two, says they can be purchased on Amazon, and says that she has tested such a product and found that its isotopic signature was -23.8‰. Given that Houlihan’s 19-NA was measured at -23‰, and thus very similar to these precursors but very different from what would be expected from pork, this is as close as Ayotte comes to offering what they believe to be the doping act in the Houlihan case.
The CAS decision gives us a really interesting insight into what World Athletics believe is happening, when Ayotte explains how a lot of positive 19-NA cases have an isotope signature around -23‰, which is very different from what would be expected from pork meat, but consistent with oral precursors that we know are sold as recovery and performance aids. So they’re quite widely available, often sold as “legal” (many people would be persuaded that a precursor is legal, when it may not be), and according to Ayotte, popping up regularly in doping controls.
So I wouldn’t dismiss as implausible the likelihood of an athlete using these substances without knowing the risk, or perhaps unknowingly, because the number of cases alone tells us that it’s not all that implausible!
it all fits nicely; shelby microdosing this precursor. maybe she wasn't fully aware. it may have been 'prescribed' to her by a coach who was not on top of the il/legality, who knows. the way she looks too manly (prolonged use of nandrolone in the body increases fat free mass and muscle strength). the way she couldn't offer any plausible defence at all. for nandrolone you don't inject it if you are looking to cheat because the detection time is way too long. you either ingest it intentionally (dope), or ingest in a contaminated supplement, or ingest in food, which was ultimately her (very weak) defence. she could have used a stronger food defence and said it was liver or heart or other organs with higher levels of nandrolone naturally present but even then the expert in the CAS ruling concludes that the levels would not be high enough and the isotope would not be the same/similar.
you have to conclude that no, contamination can’t explain the presence of 19-NA with the specific isotope signature and the level of 19-NA in this particular case.
so the reality is probably that she orally ingested that precursor in supplement form (not a contaminated supplement), and the only way she wouldn't have had knowledge of that is if somebody mixed it into her drink or whatever, or if she didn't understand what she was taking. she couldn't use the supplement defence because her substance wasn't contaminated, it was pure product.