Correct Answer: A

Rationale: The principal cation in plasma and interstitial fluid is sodium. Sodium plays a crucial role in maintaining fluid balance and is the primary cation in extracellular fluids like plasma and interstitial fluid. Potassium is the primary cation within cells, not in extracellular fluids, making it an incorrect choice. Calcium and magnesium are essential minerals but are not the principal cations in plasma and interstitial fluid, so they are also incorrect choices.

Correct Answer: C

Rationale: Scurvy is caused by a deficiency of vitamin C, not vitamin E. The symptoms of scurvy include spontaneous gingival hemorrhaging, perifollicular petechiae, follicular hyperkeratosis, diarrhea, fatigue, depression, and cessation of bone growth. Vitamin A (Choice A) is important for vision and immune function, Vitamin D (Choice B) is essential for bone health, and Vitamin K (Choice D) is necessary for blood clotting. However, none of these vitamins are associated with scurvy.

Correct Answer: D

Rationale: The oral cavity is indeed the site of various systemic disease manifestations due to multiple factors. Firstly, the rapid cellular turnover in the oral mucosa makes it susceptible to diseases. Secondly, the constant presence of microorganisms in the oral cavity contributes to the development of systemic diseases. Finally, the oral cavity being a trauma-intense environment further increases the risk of systemic manifestations. Therefore, all the provided options - rapid cellular turnover, constant attack by microorganisms, and a trauma-intense environment - play a role in making the oral cavity a site for various systemic diseases. Hence, the correct answer is 'All of the above.' Choices A, B, and C are incorrect individually as they each represent only one aspect of why the oral cavity is prone to systemic disease manifestations, whereas the correct answer encompasses all these factors.

Correct Answer: D

Rationale: Macronutrients are nutrients required by the body in large amounts to provide energy and include carbohydrates, fats, and proteins. While protein is essential for the body, it is not classified as a macronutrient. Proteins are made up of amino acids and serve various functions in the body, such as building and repairing tissues, but they are not a direct energy source like carbohydrates and fats. Therefore, protein is the exception among the listed options, making it the correct answer. Choice A, Fat, Choice B, Polyunsaturated fatty acids, and Choice C, Carbohydrate, are all examples of macronutrients that provide energy and support various bodily functions.

Correct Answer: B

Rationale: The correct answer is B, Vitamin C. Vitamin C is a water-soluble vitamin, not fat-soluble. Fat-soluble vitamins are Vitamins A, D, E, and K. These vitamins are stored in the body's fat tissues and liver, unlike water-soluble vitamins which are not stored and are eliminated in urine, making them less likely to reach toxic levels.