The correct answer is option 'B' - Renal portal system.

The venous system of a frog differs from that of a rabbit in the presence of a renal portal system. Let's understand what the renal portal system is and how it differs in frogs and rabbits.

Renal portal system:
The renal portal system is a specialized circulatory system found in some animals, including frogs. It is a unique arrangement of blood vessels that allows the blood to flow through two sets of capillary beds before returning to the heart. In simple terms, it is a system where blood passes through two sets of capillaries before reaching the heart.

In frogs, the renal portal system consists of two renal portal veins. These veins carry deoxygenated blood from the hind limbs, tail, and pelvic region to the kidneys. The blood then passes through the capillaries in the kidneys for filtration and waste removal. From the kidneys, the purified blood is collected by the renal veins and finally returns to the heart.

In rabbits, on the other hand, the renal portal system is absent. Instead, the blood from the hind limbs, tail, and pelvic region is directly carried by the veins to the inferior vena cava. The inferior vena cava is a large vein that returns deoxygenated blood from the lower body to the heart. Therefore, the blood from these regions bypasses filtration in the kidneys before returning to the heart in rabbits.

Comparison:
To summarize, the main difference between the venous system of frogs and rabbits lies in the presence or absence of a renal portal system.

In frogs:
- Renal portal veins are present.
- Blood from the hind limbs, tail, and pelvic region passes through the renal portal veins to the kidneys for filtration before returning to the heart.

In rabbits:
- Renal portal system is absent.
- Blood from the hind limbs, tail, and pelvic region directly enters the inferior vena cava and returns to the heart without passing through the kidneys for filtration.

This difference in the venous system reflects the different physiological adaptations and functions in frogs and rabbits.

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