**Core Concept**
The Glomerular Filtration Rate (GFR) is a critical measure of kidney function, reflecting the rate at which the glomeruli filter waste and excess fluids from the blood. To accurately measure GFR, an ideal substance should possess specific characteristics that enable precise calculation of filtration rate.

**Why the Correct Answer is Right**
The ideal substance for measuring GFR should not be secreted in the renal tubules. If a substance is secreted in the tubules, it will be removed from the filtrate, leading to an underestimation of GFR. This characteristic is in contrast to the ideal substance, which should be neither secreted nor reabsorbed by the renal tubules. The presence of secretion would disrupt the accurate measurement of GFR, making option A an incorrect choice.

**Why Each Wrong Option is Incorrect**
**Option B:** The substance should indeed be non-toxic to ensure safety during the measurement process.
**Option C:** The ideal substance should not be completely eliminated from the body; rather, it should be excreted in the urine in proportion to the GFR.
**Option D:** The substance should not be protein-bound, as protein binding would affect its filtration rate and accuracy of GFR measurement.

**Clinical Pearl / High-Yield Fact**
In clinical practice, inulin is often used as the gold standard for measuring GFR due to its ideal characteristics, including being neither secreted nor reabsorbed by the renal tubules.

✓ Correct Answer: A. Should be secreted in tubules