Final answer:
The best choice of heart source for a 6-month-old yorkie with a postoperative temperature of 99 deg F is the dog's own heart or an allograft from a compatible donor dog.
Step-by-step explanation:
The best heart source for a 6-month-old yorkie with a postoperative temperature of 99 deg F would be the dog's own heart or an allograft (donor) heart from another dog of the same breed and similar size.
Using the dog's own heart (autograft) would be ideal as it eliminates the risk of tissue rejection and complications associated with transplanting a foreign organ. However, if that is not possible, an allograft from a compatible donor dog would be the next best option.
It is important to ensure that the allograft is a suitable match to minimize the risk of complications and rejection by the recipient's immune system.