**Core Concept**
Chronic kidney disease (CKD) is characterized by a gradual loss of kidney function over time, leading to a decrease in glomerular filtration rate (GFR) and an accumulation of waste products in the body. Early detection and monitoring of CKD are crucial to prevent progression to end-stage renal disease (ESRD).

**Why the Correct Answer is Right**
The correct marker of CKD is serum cystatin C, which is a cysteine protease inhibitor produced by all nucleated cells. It is freely filtered by the glomerulus and neither secreted nor reabsorbed by the renal tubules, making it an ideal marker for GFR. Serum cystatin C levels are less affected by age, sex, and muscle mass compared to serum creatinine, making it a more accurate marker for CKD in various populations.

**Why Each Wrong Option is Incorrect**
* **Option A:** Serum creatinine is a commonly used marker for CKD, but it has limitations due to its dependence on muscle mass, age, and sex. It may not accurately reflect GFR in certain populations.
* **Option B:** Blood urea nitrogen (BUN) is not a specific marker for CKD and can be affected by various factors such as dehydration, gastrointestinal bleeding, and liver disease.
* **Option C:** Urine albumin-to-creatinine ratio (UACR) is a marker for kidney damage and is used to diagnose albuminuria, but it is not a direct marker for CKD.

**Clinical Pearl / High-Yield Fact**
Cystatin C is a more accurate marker for CKD in older adults and those with muscle wasting diseases, as serum creatinine levels may be artificially elevated due to muscle mass loss.

**Correct Answer:** C. Serum cystatin C.