**Core Concept**
The renal tubular function is crucial for maintaining electrolyte balance, acid-base homeostasis, and fluid status. The sensitivity of an index to renal tubular function can be evaluated by its ability to detect early changes in renal tubular transport mechanisms.

**Why the Correct Answer is Right**
The most sensitive index for renal tubular function is the **Fractional Excretion of Phosphate (FEP)**. This index is calculated as (urine phosphate x serum creatinine) / (serum phosphate x urine creatinine). FEP is highly sensitive to changes in renal tubular reabsorption of phosphate, making it an excellent marker for assessing renal tubular function. The renal tubules reabsorb most of the phosphate filtered by the glomeruli, and any impairment in this reabsorption process can lead to an increased FEP.

**Why Each Wrong Option is Incorrect**
**Option A:** The Fractional Excretion of Sodium (FENa) is a good indicator of acute kidney injury but is not as sensitive to renal tubular function as FEP. FENa is primarily used to diagnose prerenal causes of acute kidney injury.
**Option B:** The Urine-to-Serum Creatinine Ratio is a useful indicator of glomerular filtration rate (GFR) but does not directly assess renal tubular function.
**Option C:** The Urine pH is a marker of systemic acid-base status but does not specifically assess renal tubular function.

**Clinical Pearl / High-Yield Fact**
FEP is an essential index to assess renal tubular function, especially in patients with kidney diseases or those receiving nephrotoxic medications.

**Correct Answer:** C. Fractional Excretion of Phosphate (FEP).