Correct Answer: D

Rationale: The correct answer is D, 'limiting.' In the analysis of the amino acid composition of foods, the amino acid that occurs in the smallest amount is referred to as the limiting amino acid. This term is used because when one essential amino acid is deficient, it limits the synthesis of proteins, making it crucial in determining protein quality. Choices A, B, and C are incorrect. 'Indispensable' and 'dispensable' refer to the essentiality of amino acids, and 'complementary' does not describe the concept of the amino acid occurring in the smallest amount.

Correct Answer: A

Rationale: The correct answer is A: Forming neurotransmitters. Methionine, tyrosine, and tryptophan are important amino acids involved in the production of neurotransmitters in the body. Neurotransmitters are chemical messengers that transmit signals across nerve cells, influencing various functions such as mood, memory, and muscle control. Choices B, C, and D are incorrect because these amino acids do not primarily play a role in blocking protein breakdown, increasing insulin and thyroxine production, or decreasing energy expenditure.

Correct Answer: C

Rationale: Anabolism is the phase of metabolism responsible for building up and repairing tissues in the body. It involves processes that require energy to synthesize complex molecules from simpler ones. Digestion (choice A) is the process of breaking down food into simpler substances for absorption. Catabolism (choice B) involves the breakdown of complex molecules into simpler ones with the release of energy. Ketosis (choice D) is a metabolic state where the body uses fat as the primary source of energy, which is not directly related to growth and repair.

Correct Answer: C

Rationale: The correct answer is C: Small intestine. The majority of lipid digestion occurs in the small intestine, where bile and pancreatic lipase are active. Lipids are broken down into fatty acids and glycerol in the small intestine. Choices A, B, and D are incorrect because the mouth and stomach play minor roles in lipid digestion, and the large intestine is mainly responsible for absorbing water and electrolytes from the indigestible food residues.

Correct Answer: D

Rationale: The correct answer is D: By enterokinase. Trypsin is activated by enterokinase, an enzyme secreted by the small intestine. Enterokinase plays a crucial role in converting trypsinogen into its active form, trypsin. Choices A, B, and C are incorrect because trypsin is specifically activated by enterokinase, not by zymogen, bile, or gastrin.