**Core Concept**
During prolonged fasting, the body undergoes a series of metabolic adaptations to maintain energy homeostasis. Initially, glucose is the primary energy source, but as fasting progresses, the body shifts to alternative energy sources to compensate for the lack of glucose.

**Why the Correct Answer is Right**
After approximately 48 hours of fasting, the body's glycogen stores are depleted, and insulin levels are significantly reduced. In this state, the liver begins to break down stored fat into fatty acids and glycerol, which are then converted into ketone bodies through a process called ketogenesis. Ketone bodies, such as beta-hydroxybutyrate, acetoacetate, and acetone, become the primary energy source for various tissues, including the brain, heart, and skeletal muscle. This metabolic shift is mediated by the hormone glucagon, which stimulates the production of glucose from non-carbohydrate sources (gluconeogenesis) and promotes ketogenesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, so we cannot evaluate its correctness.
**Option B:** While the liver does produce glucose through gluconeogenesis during fasting, it is not the primary energy source at this stage.
**Option C:** This option is incorrect because, after 48 hours of fasting, glucose is not the main source of energy.

**Clinical Pearl / High-Yield Fact**
During prolonged fasting, the body's reliance on ketone bodies as an energy source can lead to a phenomenon known as "ketoacidosis," characterized by an excessive accumulation of ketone bodies in the blood. This condition can be life-threatening if left untreated and requires prompt medical attention.

**Correct Answer: C. Fatty acids and ketone bodies.**