A person is undergoing prolonged fasting. His urine will be found to contain abnormal quantities of :

Question

Accepted Answer

The correct option is 1: Ketones

A person undergoing prolonged fasting eventually exhausts their immediate and stored carbohydrate energy reserves, specifically the glycogen found in the liver and muscles. This depletion typically occurs within 12 to 24 hours of fasting. Once glycogen stores are low, the body enters an adaptive metabolic state to conserve protein and maintain energy supply, primarily for the brain. It achieves this by shifting its primary fuel source from glucose (carbohydrates) to lipids (fats), a process known as lipolysis. The liver begins breaking down stored triglycerides into fatty acids and glycerol. While glycerol can be used to synthesize a minimal amount of new glucose (gluconeogenesis), the fatty acids are rapidly processed in the liver to create water-soluble molecules called **ketone bodies** (ketogenesis). These ketones (acetoacetate, beta-hydroxybutyrate, and acetone) are released into the bloodstream to serve as an alternate fuel for the brain and other tissues. When the production of these ketone bodies dramatically increases due to the sustained and excessive fat breakdown during prolonged fasting, their concentration in the blood rises significantly. The kidneys filter these excess ketones, resulting in their excretion in the urine, a condition known as ketonuria, which is the expected abnormal finding.

The incorrect option is 2: Fructose

Fructose is a simple sugar, or monosaccharide, which is a type of carbohydrate. It is rapidly absorbed and metabolized by the body, primarily in the liver, and is either converted to glucose or stored as glycogen. Since prolonged fasting involves the consumption of all available carbohydrate sources to maintain blood sugar, the presence of fructose itself in the blood and urine would not be in abnormal quantities. Its appearance in urine (fructosuria) is a very rare metabolic disorder or is associated with massive ingestion, neither of which is characteristic of a fasting state. Therefore, it is not the metabolic product characteristic of this extreme catabolic process.

The incorrect option is 3: Glucose

Glucose is the body's primary energy source, and maintaining a stable blood glucose level is paramount for survival, particularly for the central nervous system. During fasting, the body actively works to prevent the loss of glucose by employing gluconeogenesis (making new glucose) from non-carbohydrate sources like amino acids and glycerol. Excreting glucose in the urine (glucosuria) occurs mainly when the blood glucose level is abnormally high, overwhelming the kidney's reabsorptive capacity, which is the hallmark of uncontrolled diabetes mellitus. Since the body is in a state of glucose conservation and low blood sugar during prolonged fasting, finding abnormal quantities of glucose in the urine is highly unlikely and would actually contradict the metabolic condition described.

The incorrect option is 4: Fats

Fats, or triglycerides, are large, complex lipid molecules that are stored in adipose tissue and are not water-soluble. When they are mobilized for energy during fasting, they are broken down into fatty acids and glycerol. The fatty acids are then metabolized *within* the liver to produce the smaller, water-soluble ketone bodies. Due to their large size and insolubility, fats are not filtered out by the kidneys and do not appear directly in the urine. Therefore, finding abnormal quantities of intact fat molecules in the urine (lipuria) is typically indicative of a severe, acute condition like a fat embolism or kidney disease, not the normal metabolic response to prolonged starvation.