**Core Concept**
The specific gravity of urine is a measure of its concentration relative to water, reflecting the kidney's ability to regulate water balance and reabsorb solutes. It is a sensitive indicator of renal tubular function, as it directly correlates with the kidney's ability to concentrate or dilute urine.

**Why the Correct Answer is Right**
The specific gravity of urine is influenced by the reabsorption and secretion of electrolytes and water in the renal tubules. It is a more sensitive indicator of renal tubular function than other measures, such as creatinine clearance or GFR, because it reflects the kidney's ability to regulate water balance and solute reabsorption. The specific gravity of urine is primarily determined by the concentration of sodium, urea, and other solutes in the urine, which is influenced by the activity of the renal tubular cells.

**Why Each Wrong Option is Incorrect**
**Option B:** Blood urea is a marker of renal function, but it is not as sensitive an indicator of renal tubular function as specific gravity of urine. Blood urea levels can be influenced by factors other than renal function, such as liver dysfunction or gastrointestinal bleeding.
**Option C:** GFR (Glomerular Filtration Rate) is a measure of renal function that reflects the kidney's ability to filter waste products from the blood. While it is an important indicator of renal function, it is not as sensitive an indicator of renal tubular function as specific gravity of urine.
**Option D:** Creatinine clearance is a measure of renal function that reflects the kidney's ability to clear creatinine from the blood. While it is a useful indicator of renal function, it is not as sensitive an indicator of renal tubular function as specific gravity of urine.

**Clinical Pearl / High-Yield Fact**
The specific gravity of urine is an important diagnostic tool in assessing renal function and tubular function. A specific gravity of 1.010 or less may indicate diabetes insipidus or other conditions characterized by excessive water loss, while a specific gravity of 1.030 or higher may indicate concentrated urine and possible dehydration.

✓ Correct Answer: A. Specific gravity of urine