High threshold substances : Such substances are absorbed almost all. Example - Sugar, amino acids, vitamins, HCO3and Na+ etc.

Non threshold substance refers to those which are not at all absorbed such as Creatinine, sulphate.

Understanding Non-Threshold Substances in Renal Reabsorption
In renal physiology, substances that are reabsorbed by the kidneys can be classified based on their threshold for reabsorption. A non-threshold substance is one that is not normally reabsorbed once it exceeds a certain concentration in the blood.
Identifying the Non-Threshold Substance
Among the options provided:
- Vitamin: Generally, vitamins such as B and C are water-soluble and may have a renal threshold. Once their concentrations exceed a certain level, they are excreted in urine.
- Keto Acid: Keto acids can be reabsorbed, but their reabsorption can also depend on their concentration and metabolic needs.
- Creatinine: This is a waste product of muscle metabolism and is typically not reabsorbed significantly by the kidneys. However, it is not classified as a non-threshold substance since its levels are usually stable and predictable.
- Amino Acid: Amino acids are actively reabsorbed in the renal tubules, but they have a threshold level. Once this level is exceeded, excess amino acids are excreted in urine.
Conclusion
The correct answer to the question of which is a non-threshold substance for renal reabsorption is none of the listed options. All substances mentioned have specific thresholds for reabsorption. However, if we focus on the concept of being minimally reabsorbed once in excess, creatinine is often considered for practical purposes due to its consistent excretion pattern.
Understanding these concepts is crucial for grasping renal function and metabolism.