Whilst doing a routine inspection of a 47-year-old man, dentists in Mexico found a weird red spot on the roof of his mouth.

The dentists were concerned about the lesion, which could have been caused by anything from blood dyscrasias (diseases of the blood) to a tumor. However, there are other ways that such lesions can occur, which aren't quite so terrifying.

After asking about the circular lesion, they found the man was unaware of it and hadn't experienced any negative symptoms. During an interview about his medical history, they found that he was a cocaine and marijuana user, though neither of these accounted for the mark. He had also tested negative for HIV, eliminating that as a possible cause.

However, they found that the man regularly performed fellatio, which could account for it. Upon further investigation, the dentists, from the University of Montemorelos, concluded that the lesion was most likely the result of "blunt trauma" received during oral sex.

In typical academic style, the case report authors begin by explaining what oral sex entails.

"Fellatio is defined as the stimulation of a man’s penis by a partner’s mouth by sucking or licking, although it can be undertaken by adults of any gender," they write in BMJ Case Reports.

"The contact of the palate with the penile glands may cause a haematoma due to blunt trauma and dilatation of the blood vessels because of the negative pressure created while sucking."

After establishing oral sex as a possible cause, the dentists sent the patient away with reassurance and asked him to return a week later. If the lesion had gone, then they could eliminate sexually transmitted infections, tumors, and other possible causes.

Sure enough, a week later the lesion had disappeared.

"We concluded the erythema on the soft palate was associated with the practice of oral sex," the authors write, stressing medical professionals should consider this explanation when making diagnoses.

"Oral sex has become a common practice in our society; therefore, it is pertinent to consider this practice as a potential cause of oral lesions, particularly on the palate."

A study cited in the case report looked at 132 sex workers and found that 17 had similar fellatio-related lesions. They can also be caused by respiratory infections, suction, anticoagulant therapy, and playing wind instruments.