## Core Concept
An **isograft** refers to a type of graft that is transplanted between two individuals who are genetically identical. This typically occurs between monozygotic (identical) twins. The underlying principle here involves immunology and transplant medicine, focusing on the genetic similarity between the donor and recipient to minimize the risk of graft rejection.

## Why the Correct Answer is Right
The correct answer, **identical twins**, is right because isografts are transferred between individuals who have the same genetic makeup. Identical twins share 100% of their genes, making them ideal candidates for organ or tissue transplantation without the concern of an immune response against the graft. This genetic identicalness eliminates the need for immunosuppressive therapy to prevent graft rejection.

## Why Each Wrong Option is Incorrect
- **Option A:** Unrelated individuals are incorrect because they do not share a similar genetic makeup, increasing the risk of graft rejection.
- **Option B:** Siblings (who are not identical twins) can share on average 50% of their genes, but they are not genetically identical, making them unsuitable for isografts.
- **Option C:** Individuals from the same species but with different genetic makeup are also incorrect for the same reason as unrelated individuals; they are not genetically identical.

## Clinical Pearl / High-Yield Fact
A key point to remember is that isografts have the lowest risk of rejection among all types of grafts because they are genetically identical. This concept is crucial in transplant immunology and is often tested in exams related to medical sciences.

## Correct Answer: D. Identical twins.