**Core Concept**
Red blood cells (RBCs) rely on anaerobic glycolysis to generate energy due to the lack of mitochondria. This process yields a small amount of ATP, which is essential for maintaining cellular functions.

**Why the Correct Answer is Right**
In RBCs, glucose is converted to pyruvate through glycolysis. The enzyme pyruvate kinase then catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate, producing one molecule of ATP in the process. This ATP is crucial for maintaining the membrane integrity and facilitating various cellular processes. The absence of mitochondria in RBCs precludes the use of fatty acids, amino acids, or ketone bodies as primary energy sources.

**Why Each Wrong Option is Incorrect**
**Option B:** Fatty acids are broken down in the mitochondria via beta-oxidation, which is not possible in RBCs due to their lack of mitochondria.
**Option C:** Amino acids are used as energy sources in other cells, but RBCs rely on anaerobic glycolysis of glucose for ATP production.
**Option D:** Ketone bodies are produced by the liver during periods of fasting or low glucose levels and are not utilized as energy sources by RBCs.

**Clinical Pearl / High-Yield Fact**
RBCs rely on anaerobic glycolysis of glucose to generate ATP, making them dependent on a continuous supply of glucose for energy.

**✓ Correct Answer: A. Glucose**