**Core Concept**
Ketone bodies are organic compounds produced by the liver from fatty acids during periods of low food intake, carbohydrate restrictive diets, prolonged intense exercise, or in uncontrolled type 1 diabetes. This metabolic process is known as ketogenesis.

**Why the Correct Answer is Right**
Ketone bodies, specifically acetoacetate, beta-hydroxybutyrate, and acetone, are produced in the mitochondria of hepatocytes through the breakdown of fatty acids. The process is initiated by the activation of hormone-sensitive lipase, which releases fatty acids from triglycerides in adipose tissue. These fatty acids are then transported to the liver, where they undergo beta-oxidation, producing acetyl-CoA. Acetyl-CoA is then converted into ketone bodies through a series of reactions involving acetoacetyl-CoA, beta-hydroxybutyryl-CoA, and acetoacetate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ketone bodies are not by-products of glucose metabolism. In fact, ketone bodies are an alternative energy source for the brain and other tissues during periods of low glucose availability.

**Option B:** This option is incorrect because lactate is a by-product of anaerobic glycolysis, not ketogenesis.

**Option C:** This option is incorrect because urea is a by-product of protein metabolism, specifically the degradation of amino acids in the liver.

**Clinical Pearl / High-Yield Fact**
Ketone bodies can serve as an alternative energy source for the brain and other tissues during periods of low glucose availability, but excessive production of ketone bodies can lead to ketoacidosis, a potentially life-threatening condition.

**Correct Answer:** D. Fatty acid metabolism