**Core Concept**
An isograft refers to tissue transplantation between individuals who are genetically identical, meaning they share the same or nearly identical human leukocyte antigens (HLA). This concept is critical in organ and tissue transplantation to minimize immune rejection.

**Why the Correct Answer is Right**
In monozygotic twins, the tissues are genetically identical due to shared embryonic origin. This results in perfect HLA matching, which significantly reduces the risk of immune rejection. Since the donor and recipient are genetically identical, the immune system recognizes the transplanted tissue as "self," making isografts the most immunologically compatible type of graft.

**Why Each Wrong Option is Incorrect**
Option A: Unrelated donors have different HLA types, leading to high immunological mismatch and significant rejection risk.
Option B: Related donors (e.g., siblings) share some HLA markers, but not all, making them non-isografts and with moderate rejection risk.
Option D: Tissue from the same individual (autograft) is not a graft between individuals, and thus does not qualify as an isograft by definition.

**Clinical Pearl / High-Yield Fact**
Isografts are rare in clinical practice due to the limited number of monozygotic twin pairs, but they represent the gold standard for immune compatibility in transplantation. Always remember: isograft ≠ autograft — the key distinction lies in whether the tissue comes from a genetically identical individual *between* two people.

✓ Correct Answer: C. Monozygotic twins