These anticoagulants themselves can lead to trouble. In a healthy person, oxygen-starved blood leaving the heart travels an intricate network of capillaries through the lungs for an oxygenating pit stop by the airways. Usually, if small fissures occur in this network, the body’s clotting agents show up to slap some circulatory duct tape on them until they heal. But for someone taking anticoagulants, the body can’t efficiently patch things up if any part of this tight blood-vessel network is breached, and things can spiral out of control.

In Wieselthaler’s case, blood eventually broke out of his patient’s pulmonary network into the lower right lung, heading directly for the bronchial tree. After days of coughing up much smaller clots, Wieselthaler’s patient bore down on a longer, deeper cough and, relieved, spit out a large, oddly shaped clot, folded in on itself. Once Wieselthaler and his team carefully unfurled the bundle and laid it out, they found that the architecture of the airways had been retained so perfectly that they were able to identify it as the right bronchial tree based solely on the number of branches and their alignment.

“We were astonished,” Wieselthaler says. “It’s a curiosity you can’t imagine—I mean, this is very, very, very rare.”

It’s rare, but not entirely unprecedented. A case study that appeared in The Journal of the American Medical Association back in 1926 describes a 34-year-old woman who was admitted to Rochester Municipal Hospital with an airway infection and coughed out “a large piece of membrane”—a layer of cells and gunk collected by the infection—“which proved to be a cast of the trachea, both bronchi, and several bronchioles.” In September 2005, the European Journal of Cardio-Thoracic Surgery published a photo of a smaller bronchial-tree cast coughed up by a pregnant 25-year-old who had developed a disorder impairing her clotting agents. She recovered and was able to deliver a healthy infant. The woman with the airway infection, who was born before the diphtheria vaccine, was not as lucky.

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The primarily pediatric condition plastic bronchitis, a lymphatic-flow disorder associated with various heart and lung diseases, causes a buildup of lymph fluid in the airways that becomes hard and rubbery, which many people cough up in a similarly pristine structure. And for asthmatics, mucus plugs can harden in the airways thanks to factors like bronchoconstriction and dehydration, making them ready to be coughed up during an asthma attack.

Still, for all these cases, only the mother-to-be coughed up a cast made of blood, the largest ever photographed until UCSF’s. Congealed blood is less sturdy and sticky than hardened lymph or mucus, so why didn’t the cast break apart?

Wieselthaler suspects the answer might involve fibrinogen, a protein component of blood plasma that essentially acts as the “glue” of a clot by trapping platelets to form a mass. The infection that Wieselthaler’s patient had, in addition to aggravating his heart failure, caused a higher-than-normal concentration of fibrinogen in his blood. It’s possible, Wieselthaler says, that the blood in his airways was unusually rubbery, capable of surviving the bumpy ride up the trachea unscathed.