**Core Concept**
The question is testing the understanding of saphenous vein graft (SVG) failure following coronary artery bypass grafting (CABG). SVG failure is a major concern in patients undergoing CABG, leading to recurrent ischemia and potential need for repeat revascularization.

**Why the Correct Answer is Right**
The most frequent cause of SVG failure is intimal hyperplasia, a process characterized by excessive proliferation and migration of smooth muscle cells, leading to thickening of the intimal layer and subsequent luminal narrowing. This is a result of the vein's endothelial surface being exposed to arterial pressure and flow, leading to a healing response that ultimately causes the vein's lumen to narrow. The process is mediated by various growth factors, including platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-β), which promote smooth muscle cell proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** Atherosclerosis is a common cause of SVG failure, but it is not the most frequent cause. Atherosclerosis is a process that affects the entire arterial tree, including native coronary arteries, and is not specific to SVGs.

**Option B:** Thrombosis is a potential complication of SVG failure, but it is not the most frequent cause. Thrombosis can occur in conjunction with intimal hyperplasia, but it is a secondary event rather than the primary cause of SVG failure.

**Option C:** Infection is a rare complication of CABG, and is not a common cause of SVG failure. Infection can lead to graft failure, but it is not the most frequent cause.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that SVG failure is often asymptomatic in the early stages, and may only become apparent during follow-up imaging or stress testing. This highlights the importance of regular surveillance and monitoring of patients post-CABG.

**Correct Answer: C. Intimal hyperplasia.**