Doctors have told of their surprise after discovering a Portuguese woman they were examining had two anal canals.
The unidentified patient was referred to specialists after enduring two abscesses on her bottom, which both required 'drainage'.
Upon closer inspection, medics in Figueria da Foz, saw two holes leaving her anus - instead of the usual one.
The 40-year-old woman refused numerous offers of surgery to correct her defect, which medics say she would have been born with.
Writing in the
Journal of Medical Cases, they revealed anal canal duplication is 'the most uncommon' malformation of the digestive tract.
They wrote: 'This case is of particular importance because of its rarity. Around 60 cases of ACD are currently described worldwide.
'Our patient was diagnosed at the age of 40 years, constituting one of the most elderly patient diagnosed with ACD.'
They revealed that 'diagnosis is usually made in the first year of life by caregivers'.
Faecal matter passes through the anal canal, which connects directly to the rectum - where waste is stored before it leaves the body.
In theory, the woman could have been able to excrete through both, if her second canal linked to the rectal wall, which it didn't.
Instead, doctors revealed the 'tubular and cul-de-sac' shaped orifice, separated from the other canal by just skin, was just 3cm deep and 1cm wide.
Hospital Distrital da Figueira da Foz explained how the woman came to them after she reported two anal abscesses that needed surgically draining in the past year.
She was referred for a consultation with proctologists, led by Dr Jessica Neves - despite no longer having any painful pus-filled lumps.
Examination revealed the two orifices, and medical scans to help doctors identify the problem confirmed it was a duplication of the anal canal.
Doctors asked the patient if she wished to undergo surgery to correct her defect at the time, but she refused.
Seven years later she returned to the same clinic complaining of a new abscess on her bottom and was again offered surgery. However, she again declined.