Correct Answer: C. Isograft
An isograft is a tissue or organ transplant between genetically identical individuals—specifically, identical twins (monozygotic twins). Since identical twins share 100% of their DNA, the graft is recognized as "self" by the recipient's immune system, eliminating the risk of rejection. This is the only type of allograft that does not require immunosuppression. In clinical practice, isografts are the gold standard for transplantation when available, as they achieve the best long-term outcomes without the morbidity of chronic immunosuppressive therapy. The term "iso-" means "same" or "equal," referring to genetic identity. While syngeneic grafts (between genetically identical non-twin individuals) are theoretically similar, the clinical definition of isograft specifically refers to identical twin transplants. This distinction is critical in Indian medical education and NEET PG examinations, where the terminology must be precise.
Why the other options are wrong
A. Allograft — An allograft is a transplant between genetically different individuals of the same species. While identical twins are technically the same species, they are genetically identical, not different. Allografts trigger immune rejection and require immunosuppression. This is the most common type of clinical transplant (kidney, heart, liver), but it does NOT apply to identical twin grafts. NBE may trap students who confuse 'same species' with 'same genetics.' B. Xenograft — A xenograft is a transplant between different species (e.g., pig heart to human, bovine valve to human). Xenografts trigger the strongest immune response due to major species differences and are used only in emergency situations or as temporary bridges. They are completely unrelated to identical twin transplants. This option tests whether students understand species-level classification in transplantation. D. Allograft — This is a duplicate of option A and is incorrect for the same reason: allografts are between genetically different individuals of the same species. The duplication itself may be a transcription error in the original question, but the answer remains wrong. Students must recognize that genetic identity (isograft) is fundamentally different from genetic difference (allograft).
High-Yield Facts
- Isograft = transplant between genetically identical twins; 0% rejection risk; no immunosuppression needed.
- Allograft = transplant between genetically different individuals of same species; requires immunosuppression; most common clinical transplant type.
- Xenograft = transplant between different species; strongest immune rejection; used only as temporary bridge in emergency.
- Autograft = transplant from one site to another on same individual; no immune rejection; e.g., skin graft, bone graft.
- Identical twins share 100% DNA identity; fraternal twins share only ~50% and are NOT candidates for isografts.
- In India, living-related kidney transplants (parent-to-child, sibling-to-sibling) are allografts, not isografts, unless donor and recipient are identical twins.
Mnemonics
AXIA Classification Autograft (same person), Xenograft (different species), Isograft (identical twins), Allograft (same species, different genetics). Use this to lock in the four main graft types in order of immunological compatibility. Iso = Identity Iso- prefix means 'same' or 'equal' → Isograft = same genetics = identical twins. Contrast with allo- (other) and xeno- (foreign).
NBE Trap
NBE pairs "identical twins" with "allograft" to trap students who memorize "allograft = same species" without recognizing that genetic identity is the discriminating feature. The duplicate option D reinforces this trap by making students second-guess whether they're confusing terminology.
Clinical Pearl
In Indian transplant centers, identical twin kidney transplants are rare but represent the ideal scenario—patients achieve indefinite graft survival without the long-term toxicity of calcineurin inhibitors or mTOR inhibitors. A patient with an isograft can discontinue all immunosuppression after 1–2 years, unlike allograft recipients who remain on triple therapy for life.
_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 6 (Diseases of Immunity); Harrison's Principles of Internal Medicine, Ch. 135 (Transplantation)_