**Core Concept**
Hyperacute graft rejection is an immediate and severe form of rejection that occurs within minutes to hours after transplantation due to pre-existing antibodies against the graft. This is often seen in kidney transplants and is caused by the presence of donor-specific antibodies (DSAs) in the recipient's blood. The underlying mechanism involves the activation of the complement system and the destruction of the endothelial cells lining the graft vessels.

**Why the Correct Answer is Right**
The correct answer is **A. Pre-existing donor-specific antibodies**. This occurs when the recipient has pre-formed antibodies against the HLA (Human Leukocyte Antigen) antigens present on the donor's graft. These antibodies can bind to the HLA antigens on the graft's endothelial cells, leading to the activation of the complement system. The subsequent formation of the membrane attack complex (MAC) causes the destruction of the endothelial cells, resulting in thrombosis and ischemia of the graft.

**Why Each Wrong Option is Incorrect**
**Option B:** Hyperacute rejection is not caused by immediate immune system activation in response to the graft. While it is an immune-mediated process, it is not an immediate response to the graft.
**Option C:** While inflammation and cellular immune responses are involved in graft rejection, hyperacute rejection is a distinct entity driven by pre-existing antibodies.
**Option D:** Hyperacute rejection is not caused by the recipient's immune system recognizing the graft as foreign and initiating an immune response. Instead, it is caused by pre-existing antibodies against the graft.

**Clinical Pearl / High-Yield Fact**
To prevent hyperacute rejection, it is essential to perform cross-matching between the donor's blood and the recipient's serum to detect pre-existing DSAs. This can be done using various techniques, including lymphocytotoxicity assays and flow cytometry.

**Correct Answer: A. Pre-existing donor-specific antibodies**