Possible Causes for Red Urine with Bleach Addition?

[mintymurs]

Can anyone help me establish what is the probable cause of human urine turning a dark reddish colour upon the addition of an amount of bleach?

Big thank you.

mintymurs

 

[Borek]

Perhaps effect of urobilinogen being oxidized to urobilin. Both are always present in the urine, the latter is much darker.

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[chemisttree]

I wonder if it could be some anthraquinones from the diet or from an herbal laxative.
In the US before 1999, phenolthalein was used as the active ingredient in Ex-Lax. I'm not sure if phenolphthalein is still in use in the UK (Australia?) but if it is and you used it, it can find it's way into the urine.
Same goes for Correctol or bisacodyl. This structure is farily easily hydrolyzed and/or oxidized to a hydroquinone.