## Core Concept
The question tests the understanding of **immunogenetics** and **histocompatibility** in the context of organ transplantation, specifically focusing on the terms **isograft**, **allograft**, **syngraft**, and **autograft**. The key concept here is the genetic relationship between the donor and the recipient and its impact on graft compatibility.

## Why the Correct Answer is Right
The correct answer, **allograft**, refers to a graft of tissue obtained from a donor of the same species as, but with a different genetic makeup from, the recipient. In the case of a kidney transplant from a mother to her son, despite being related, they are not genetically identical. This makes the transplant an example of an **allograft** because the donor and recipient are **genetically distinct** but belong to the same species.

## Why Each Wrong Option is Incorrect
- **Option A: Isograft** - This is incorrect because an isograft refers to a graft from one part of the body to another in the **same individual** or between **genetically identical individuals** (like twins). A mother and son are not genetically identical.
- **Option B: Syngraft** - This term is essentially synonymous with isograft and refers to grafts between genetically identical individuals. Therefore, it does not apply to a transplant between a mother and her son.
- **Option D: Autograft** - This is incorrect because an autograft (or autotransplant) involves transplanting tissue from one part of the body to another in the **same individual**, which does not apply to this scenario.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **allografts** are the most common type of transplant and require ** immunosuppression** to prevent rejection because the donor and recipient are not genetically identical. This is in contrast to **isografts/syngrafts**, which do not require immunosuppression due to the genetic identicalness of the donor and recipient.

## Correct Answer: . Allograft