**Core Concept**
Graft arteriosclerosis is a chronic and progressive form of vascular rejection that affects the allograft vasculature, leading to luminal narrowing and eventual graft failure. This condition is characterized by the deposition of fibrous and fatty tissue within the arterial walls, which compromises blood flow and oxygen delivery to the graft.

**Why the Correct Answer is Right**
Graft arteriosclerosis is the major limitation to long-term success in cardiac transplantation due to its insidious onset and irreversible nature. This condition is mediated by the immune response to the allograft, which triggers a cascade of events involving T-cell activation, cytokine release, and the deposition of extracellular matrix proteins. The involvement of both cellular and humoral immunity contributes to the development of graft arteriosclerosis, making it a formidable challenge for clinicians to manage. The histopathological hallmark of graft arteriosclerosis is the presence of intimal thickening, which can lead to luminal narrowing and eventual graft failure.

**Why Each Wrong Option is Incorrect**
**Option A:** Allograft rejection is a broader term that encompasses acute and chronic rejection, but it is not the specific complication that currently limits long-term success in cardiac transplantation.

**Option C:** Graft atherosclerosis is a form of vascular disease that affects the allograft, but it is not the major limitation to long-term success in cardiac transplantation. While atherosclerosis can contribute to graft failure, it is not as prevalent or insidious as graft arteriosclerosis.

**Option D:** Opportunistic infections are a significant concern in immunocompromised patients, but they are not the major limitation to long-term success in cardiac transplantation.

**Clinical Pearl / High-Yield Fact**
Graft arteriosclerosis can be distinguished from other forms of rejection by the presence of intimal thickening and the absence of significant inflammation. Clinicians should be aware of this distinction to provide timely and effective management.

**✓ Correct Answer: B. Graft arteriosclerosis**