Which of the following is the distinguishing characteristic between the venous systems of frogs and rabbits?

The venous system is the system of veins in the circulatory system that collects deoxygenated blood from the rest of the body and returns it to the heart.

The distinguishing characteristic between the venous systems of a frog (amphibian) and a rabbit (mammal) is the renal portal system. 

The renal portal system is extensively developed in amphibians which brings blood from the internal organs to their kidneys whereas, it is absent in mammals for example, rabbits.