What is "renal threshold"?
(a) The highest concentration of substances up to which it is totally reabsorbed from glomerular filtrate.
(b) At which all the substances are reabsorbed.
(c) At which the filtration of a substance starts.
(d) At which no substance is filtered in the glomerulus

Options

A. The highest concentration of substances up to which it is totally reabsorbed from glomerular filtrate.
B. At which all the substances are reabsorbed.
C. At which the filtration of a substance starts.
D. At which no substance is filtered in the glomerulus

Answer

To answer the question "What is 'renal threshold'?", we can break down the definition and the options provided step by step.

### Step-by-Step Solution:

1. **Understanding Renal Threshold**:
- The renal threshold refers to the maximum concentration of a substance in the blood that can be completely reabsorbed by the kidneys. If the concentration exceeds this threshold, the kidneys cannot reabsorb all of it, and some will be excreted in urine.

2. **Analyzing the Options**:
- **Option (a)**: "The highest concentration of substances up to which it is totally reabsorbed from glomerular filtrate."
- This option accurately describes renal threshold, as it indicates the maximum concentration that can be fully reabsorbed.

- **Option (b)**: "At which all the substances are reabsorbed."
- This is incorrect because not all substances are reabsorbed; only those below their respective renal thresholds are.

- **Option (c)**: "At which the filtration of a substance starts."
- This is misleading, as filtration occurs continuously; renal threshold specifically refers to reabsorption limits, not filtration initiation.

- **Option (d)**: "At which no substance is filtered in the glomerulus."
- This is incorrect because substances are filtered in the glomerulus regardless of renal threshold.

3. **Conclusion**:
- Based on the analysis, **Option (a)** is the correct answer. It correctly defines renal threshold as the highest concentration of a substance that can be completely reabsorbed from the glomerular filtrate.

### Final Answer:
**(a) The highest concentration of substances up to which it is totally reabsorbed from glomerular filtrate.**