**Core Concept**
A xenograft refers to a type of graft that involves transplanting tissue or an organ from one species to another. This can include transplanting organs or tissues from animals to humans, or vice versa.

**Why the Correct Answer is Right**
Xenografts are used in various medical applications, including organ transplantation and tissue engineering. The primary challenge with xenografts is the risk of rejection due to the immune system's recognition of the foreign tissue. To overcome this, researchers have been exploring the use of immunosuppressive agents and genetic modification techniques to reduce the risk of rejection. In addition, xenografts can also be used to study human diseases in animal models, such as the use of pig hearts in non-human primates to study human cardiac function.

**Why Each Wrong Option is Incorrect**
**Option A:** Autograft - Incorrect because an autograft involves transplanting tissue or an organ from one part of the body to another, whereas a xenograft involves transplanting tissue or an organ from one species to another.
**Option B:** Allograft - Incorrect because an allograft involves transplanting tissue or an organ from one individual to another of the same species, whereas a xenograft involves transplanting tissue or an organ from a different species.
**Option C:** Isograft - Incorrect because an isograft involves transplanting tissue or an organ from one individual to another who is genetically identical, whereas a xenograft involves transplanting tissue or an organ from a different species.

**Clinical Pearl / High-Yield Fact**
It's essential to note that xenografts can pose a risk of transmitting zoonotic diseases, such as the transmission of porcine endogenous retroviruses (PERVs) from pigs to humans. This highlights the need for careful screening and testing of xenografts before transplantation.

**Correct Answer: C. A graft of tissue or organ from one species to another.