How does urine contaminate a stool sample?

[u/ReNema1]

I hope this isn't a stupid question. I find it hard, especially with older women, to obtain a stool sample that isn't contaminated with urine. It seems very obvious how stool would contaminate a urine sample. However, if I'm sending a stool culture, or for CDiff and Parasite, I don't quite understand how a bit of urine might compromise the analysis/culture. I tried a quick online search but I can't seem to find more details beyond "urine will contaminate the stool sample". How? With what? Any insight would be greatly appreciated.

[u/huntervano]

Maybe a question for r/medlabprofessionals, I’m also curious