Which of the following substances is not usually present in the urine of a healthy person?

Step 1: Analyze the question We need the substance that is not normally found in the urine of a healthy person.

Step 2: Explain the concept The kidneys filter blood in the nephrons. Filtration happens in the Bowman's capsule, and the filtrate contains water, glucose, urea, mineral salts and amino acids. Then selective reabsorption takes place, mainly in the proximal convoluted tubule:

• All glucose and amino acids are reabsorbed back into the blood by active transport.
• Most water and useful salts are reabsorbed as the body needs them.
• Urea, excess salts and excess water stay in the filtrate and leave as urine.

Step 3: Check each option

• A. Urea: This is the main nitrogenous waste, so it is present in urine.
• B. Mineral salts: Excess salts such as sodium chloride are excreted, so they are present.
• C. Glucose: It is completely reabsorbed in a healthy person, so it is normally absent from urine.
• D. Water: It is the major component of urine (about 95%), so it is present.

Step 4: Answer The correct answer is C. Glucose.

Glucose in urine (glycosuria) is a sign of a problem, such as diabetes mellitus, where blood sugar is so high that the kidney cannot reabsorb all of it.

Exam tip: Remember that "healthy urine has no sugar and no protein." If a question asks what is absent from normal urine, think glucose, proteins, and blood cells. Doctors test for these to detect diabetes and kidney disease.