Correct Answer: B

Rationale: The correct answer is B. The classification of the 20 common amino acids is not based on the properties of their side chains, but rather on the structure of their amino and carboxyl groups. These groups are consistent among all amino acids, while the side chains, also known as R groups, are what vary and give each amino acid its unique properties. Choice A is correct because proteins are indeed composed of amino acids. Choice C is incorrect because while the side chains of amino acids can vary, the general structure of an amino acid - a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a side chain - is consistent. Choice D is correct because amino acids do polymerize to form long chains called polypeptides, which can then fold into complex shapes to form proteins.

Correct Answer: B

Rationale: Malnutrition is a common comorbidity in patients with Chronic Kidney Disease (CKD). This is mainly due to factors such as dietary restrictions, poor appetite, and the body's increased nutritional needs as it struggles to deal with the disease. Liver disease (Choice A) is not typically associated directly with CKD, although both conditions may coexist in some patients. Acute renal failure (Choice C) is not a comorbidity but a severe and potentially lethal progression of CKD. Difficulty breathing (Choice D) is not a comorbidity but can be a symptom of severe kidney disease or other underlying conditions. However, malnutrition is more commonly observed in CKD patients compared to difficulty breathing.

Correct Answer: C

Rationale: Glucose is the primary sugar used by the body for energy and is converted from other sugars during metabolism. Sucrose is a disaccharide composed of glucose and fructose, not the end product of sugar metabolism. Fructose is a simple sugar found in fruits but needs to be converted to glucose for cellular energy. Maltose is a disaccharide composed of two glucose units and is not the final product of sugar metabolism in humans.

Correct Answer: C

Rationale: The correct answer is vitamin E. Vitamin E is the major antioxidant found in cell membranes, where it plays a crucial role in protecting them from oxidative damage. Although B12, beta-carotene, and vitamin A are important nutrients with specific functions in the body, they are not the primary antioxidants found in cell membranes. Vitamin E specifically localizes in cell membranes to neutralize free radicals and prevent lipid peroxidation, making it an essential antioxidant for cellular health.

Correct Answer: D

Rationale: The correct answer is D. Polyunsaturated fatty acids. Polyunsaturated fatty acids have two or more double bonds in their carbon chains, making them less stable and more prone to oxidation. This characteristic distinguishes them from saturated fatty acids (Choice A), which have single bonds between carbon atoms providing stability. Monounsaturated fatty acids (Choice B) contain only one double bond, and trans fatty acids (Choice C) can have varying numbers of double bonds but are not specifically defined by having two or more double bonds.