Urine of a person contains abnormal quantities of ________ during prolonged fasting

The correct option is ketones.

During prolonged fasting or starvation, the body undergoes a series of metabolic adaptations to maintain energy homeostasis when dietary glucose is unavailable. Here is a step-by-step breakdown of why ketones appear in abnormal quantities in the urine:

1. Depletion of Glycogen Stores:
Initially, the body relies on glucose stored in the liver and muscles as glycogen. However, these glycogen reserves are highly limited and are typically depleted within 12 to 24 hours of fasting.

2. Shift to Fat Metabolism (Lipolysis):
Once glycogen is exhausted, the body shifts its primary energy source to stored lipids (fats). Adipose tissue breaks down triglycerides into glycerol and free fatty acids. These fatty acids are transported to the liver to be broken down via beta-oxidation into acetyl-CoA.

3. Ketogenesis in the Liver:
Under normal conditions, acetyl-CoA enters the citric acid (Krebs) cycle by combining with oxaloacetate. However, during prolonged fasting, oxaloacetate is diverted to gluconeogenesis (the synthesis of glucose from non-carbohydrate sources) to supply glucose to glucose-dependent cells like red blood cells. Because of this, acetyl-CoA accumulates in the liver. The liver then converts this excess acetyl-CoA into water-soluble molecules called ketone bodies (such as acetoacetate, beta-hydroxybutyrate, and acetone) in a process called ketogenesis.

4. Excretion in Urine (Ketonuria):
The liver releases ketone bodies into the bloodstream to be used as an alternative fuel source, especially by the brain. When the production of ketone bodies by the liver exceeds the rate at which peripheral tissues can consume them, their level rises in the blood (ketonemia). The kidneys filter the blood, and the excess ketones are excreted in the urine. This presence of elevated ketones in urine is known as ketonuria, which is a classic biochemical marker of prolonged fasting.