I haven't said my piece about this, here it goes:
1- It has to be someone that has access to the MC's room.
2- It has to be someone that if found had a good reason to be there.
3- It has to be someone that wouldn't mind leaving that in plain view (never mind even buying it).
4- It has to be someone that the MC creampied at least 2 weeks ago (Tests OTC aren't reliable before then).
5- It has to be someone that is feeling some type of symptoms to even consider having the test.
6- It has to be someone that would know that the MC would look and wouldn't care if he did.
Point 1, clears up everyone outside the house.
Point 2, clears up everyone not Living in the house and not just a visit.
Point 3, clears up Asami and Shizuka. Not only would they die of shame before buying it, they would probably bury it if they had used it. While it could also clear Kat, she managed to live on her own in College, so, probably, she's more independent.
Point 4, clears no one of the in house women that are possible.
Point 5, this is the point that remains in MB's hands and allows him to spring anyone into this.
Point 6, only Liz would truly be like this. All the others would want to make a BIG THING out of it.
So, my conclusion is that it is probably Liz, but, MB has enough room to choose whomever he wants. Heck, he could choose anyone and justify it somehow, or, not even that.
To the best of my logic, I go with Liz. But I am completely OK that MB can put anyone there. Anyone. Even Allison. So, there it is.
Peace
