**Core Concept**
Chronic Kidney Disease (CKD) or Chronic Renal Failure (CRF) is a gradual loss of kidney function over time. The clinical features of CRF become apparent when renal function is significantly impaired, affecting various bodily systems.

**Why the Correct Answer is Right**
The clinical features of CRF appear when renal function is reduced to about 25-30% of normal. At this point, the kidneys are no longer able to effectively filter waste products, regulate electrolyte balance, and maintain acid-base equilibrium. This leads to the accumulation of uremic toxins, which cause various systemic symptoms such as nausea, vomiting, fatigue, and anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because renal function needs to be significantly impaired to manifest clinical features of CRF. A mild reduction in renal function may not be symptomatic.
**Option B:** Incorrect because the exact percentage of renal function loss is not a clear-cut threshold, and it can vary from person to person.
**Option C:** Incorrect because CRF is a gradual process, and clinical features do not appear suddenly or at a specific point.

**Clinical Pearl / High-Yield Fact**
The clinical features of CRF can be remembered by the mnemonic "UREMIC": Uremia, Respiratory symptoms, Electrolyte imbalance, Metabolic acidosis, Intestinal symptoms, Cardiac effects, and Neurological symptoms.

**Correct Answer:** C.