How do you unlock kidney stone?

[Marsel]

Wiki says ,,Beat challenge #29 (Added in Afterbirth / Removed in Afterbirth † The unlock is bugged and actually unlocks Blank Rune)'' so how do you do it?

[Memo]

I don't think it's true. I did Onan's Streak last week and I'm 100% sure I got Kidney Stone, not blank rune. I'd say just go for it.

[Nazemec]

Originally posted by Marsel:

Wiki says ,,Beat challenge #29 (Added in Afterbirth / Removed in Afterbirth † The unlock is bugged and actually unlocks Blank Rune)'' so how do you do it?

Blank Rune and Kidney Stone unlocks are swapped in Afterbirth. This got fixed in Afterbirth+.

So if you only have Afterbirth, check the requirement of the Blank Rune and do that, you'll get the Kidney Stone.
If you have Afterbirth+, read what you posted. Beat challenge 29.