Final answer:
An unexplained elevation of prothrombin time (PT) in a 72-yr-old smoker with COPD is most likely due to inadequate production of clotting factors.
Step-by-step explanation:
An unexplained elevation of prothrombin time (PT) in a 72-yr-old smoker who has been diagnosed with chronic pulmonary obstructive disease (COPD) is most likely due to inadequate production of clotting factors. COPD can lead to a decrease in the production of clotting factors in the liver, which can result in a prolonged PT. This is because clotting factors are produced in the liver and any liver damage or dysfunction can affect their production.