Final answer:
The posteromedial papillary muscle receives blood supply from the right bundle branch, which ensures simultaneous contraction with the rest of the ventricle. The left ventricle is supplied by the left bundle branch.
Step-by-step explanation:
The blood supply for the posteromedial papillary muscle comes from the right bundle branch, which is part of the electrical conduction system of the heart. The right bundle branch supplies the right ventricle, and portions of it are found in the moderator band, which is a muscular ridge in the right ventricle. This connection allows the impulse to reach the posteromedial papillary muscle at approximately the same time as the rest of the ventricle, ensuring simultaneous contraction. The left ventricle, on the other hand, receives blood supply from the left bundle branch.