Hong Kong doctors have uncovered the unlikely cause of a man's abdominal swelling: a massive ovarian cyst.

The 66-year-old patient, who was raised as a man because of what doctors called a "micropenis," had two rare genetic disorders - one that masked the female features he should have had, and one that gave him male features in the absence of a Y chromosome.

In fact, the patient had only one sex chromosome rather than the usual two, and it was an X.

"Were it not due to the huge ovarian cyst, his intriguing medical condition might never have been exposed," doctors who removed the cyst wrote in the Hong Kong Medical Journal.

Roughly one in 2,500 people is born with one X chromosome and no Y, a genetic disorder dubbed Turner syndrome. They are genetically and physically female, but their reproductive systems fail to fully develop.

In this case, the patient also had a genetic enzyme deficiency - one that affects fewer than one in 10,000 people. The disorder, called 21-OHD, caused an increase in androgen levels leading to the misplaced male features.

The odds of having both disorders are about one in 25 million. Only six other cases have ever been reported, according to the case study.

The patient decided to keep his male gender identity after having his ovaries, uterus and fallopian tubes removed.