**Core Concept**
Glycogen is the primary storage form of glucose in the liver and muscles, serving as the main source of readily available energy during metabolic demands, especially in the absence of immediate glucose availability.

**Why the Correct Answer is Right**
Glycogen is broken down into glucose-1-phosphate via glycogen phosphorylase, which is then converted to glucose-6-phosphate and enters glycolysis to generate ATP. In the liver, glycogenolysis provides glucose to maintain blood sugar levels; in muscle, it fuels contraction. Unlike fat or ketones, glycogen is rapidly mobilized and directly used for energy without requiring metabolic conversion. Lactate is a byproduct of anaerobic glycolysis and can be converted to glucose via gluconeogenesis but is not a primary energy source.

**Why Each Wrong Option is Incorrect**
Option A: Fat requires extensive breakdown (lipolysis, β-oxidation) and is a long-term energy source, not the immediate primary source.
Option C: Lactate is a metabolic end-product, not a direct energy source; it must be converted to glucose via gluconeogenesis.
Option D: Ketones are alternative fuels in fasting or low-carb states, but they are not the main source in normal physiological conditions.

**Clinical Pearl / High-Yield Fact**
In hypoglycemia, the liver rapidly breaks down glycogen to release glucose—this is why glycogen is the body’s first-line energy reserve, especially in the brain and muscles.

✓ Correct Answer: B. Glycogen