A story in the New York Times Magazine told of a teenage girl from New Jersey who was studying with her friends and laid down on the ground as she felt she was going to become unconscious saying that it was a common occurrence, and as she laid there, a puddle of blood formed around her coming from her mouth and nose, her friends were not able to find a pulse and ended up alling the nurse and 911. As mentioned before, this was not the first time this girl had the experience, as it happened a few months earlier.
After meeting with a cardiologist and wearing a heart monitor for a month, the results came back normal but the cardiologist decided it would be best to implant a defibrillator on her heart in case it happened a third time. Meanwhile, multiple tests were done to find out where the blood was coming from because there was no evidence of anything that would cause her bleeding in her Upper GI tract or her lungs. She was in the hospital for about a week before anything was found. Dr. Tej Phatak, who was a pediatrician and radiologist for the patient saw an irregularly shaped splash of brightness in the bottom of her right lung, which he immediately recognized as pulmonary arteriovenous malformation (PAVM), which is an abnormal connection between a pulmonary vein and an artery, which are prone to tearing which can cause dangerous bleeding. The patient was sent to Yale-New Haven Hospital to have surgery to have the PAVM closed up by a coiled wire where blood would clot, sealing off the vessel.
This abnormality is often seen in patients with hereditary hemorrhagic telangienctasia (HHT), which often causes abnormal blood vessels to form and is normally inherited from the parents, but genetic testing showed that this teenage girl has the mutation while her parents do not, meaning that the mutation happened after conception.
Article: Why Was This Teenager Bleeding So Excessively?
More information: What is HHT?