The primary threshold substances are …..........
A. Urea and uric acid
B. Potassium and calcium
C. Glucose and amino acids
D. Plasma proteins

Answer Verified Verified
Hint: Any material in the body, that after being processed gets removed from the body by the process of excretion is known to be in transit. Now, in the urine when the plasma concentration increases after a defined amount, such substances are referred to as a threshold substance.

Complete answer: A substance present in blood is filtered through kidneys. Further, depending on the concentration of the material being excreted, they can be categorized as follows: high threshold substances, low threshold substances, and lastly no-threshold substances.
Substances such as chlorides or glucose are high threshold substances; substances such as phosphates or urea are low threshold substances and creatinine sulfates considered to be a non-threshold substance.
The last category of metabolites is called so as they are not excreted in the urine. Among the components of urine, the major part comprises urea (in mammals) and uric acids (in lower animals).
As a result, they are considered to be the primary threshold substances.

So, the correct answer is option A. Urea and uric acid.

Note: It is important to note that the renal threshold varies from one species to another. It is affected by several physiological factors. The threshold value can even be altered by administering a person to drugs. The renal threshold value is essential in determining the status of a person’s kidney in terms of its functioning.