Rationale

C is correct: Lipids, specifically triglycerides stored in adipose tissue, are the body's primary long-term energy reserve. Fat yields approximately 9 kcal/gram compared to 4 kcal/gram for carbohydrates or protein, making it the most energy-dense storage form. A is incorrect: Glycogen is the short-term carbohydrate storage form found in the liver and muscles. It is rapidly mobilized but stores are limited (approximately 400-500 g total). B is incorrect: Proteins are structural and functional molecules; the body catabolizes protein for energy only when carbohydrate and fat stores are depleted. D is incorrect: Nucleic acids (DNA, RNA) carry genetic information; they are not energy storage molecules.

Key concepts