While doctors still aren't 100 per cent sure how the clot stayed completely intact on its way up the man's throat, Dr Wieselthaler suspected it was because the man had more fibrinogen in his blood.
It came from a 36-year-old man with end-stage heart failure who spat out the medical anomaly in one piece during a an extreme bout of coughing in hospital.
Despite the best efforts of intensive care staff at the University of California San Francisco Medical Centre, the man died a week later.
The patient, who had a history of heart problems, had been placed on oxygen and given blood thinners to help circulate blood and prevent clogging.
During his hospitalization, the man started to cough up phlegm and blood, and at one point it had the shape of the "intact cast of the right bronchial tree".
A 36-year-old man was admitted to the ICU with an acute exacerbation of chronic #heartfailure.
"We were astonished", Dr Wieselthaler said.
The clot shows the three segmental branches in the upper lobe (white arrows), two segmental branches of the middle lobe (black arrows) and five segmental branches of the lower lobe (blue arrows), the NEJM said. The patient's trachea was subsequently intubated, and flexible bronchoscopy revealed a small amount of blood in the basilar branches of the right lower lobe.
Bronchial tree clots (called casts) are actually relativelycommon but only in extremely rare cases do they emerge in such pristine condition.
After this episode, he ceased coughing up blood, and his tubes were removed a couple days later.
The patient was later extubated and "had no further episodes of hemoptysis", the doctors wrote, but a week later he unfortunately "died from complications of heart failure (volume overload and poor cardiac output) despite placement of the ventricular assist device".