Correct Answer: D

Rationale: The corrected answer is D, 'All of the above.' The urinary system performs multiple essential functions to maintain the body's internal environment. Synthesizing calcitriol and secreting erythropoietin are crucial roles of the kidneys. Regulating the concentrations of sodium, potassium, chloride, calcium, and other ions helps in maintaining the electrolyte balance. Reabsorbing or secreting hydrogen ions and bicarbonate is vital for managing the body's acid-base balance. Furthermore, the urinary system is involved in detecting reductions in blood volume and pressure, aiding in the maintenance of blood pressure levels and fluid balance in the body.

Correct Answer: B

Rationale: Responsiveness is the correct term that refers to the ability to detect and respond to changes in the internal and external environment. In this context, growth is incorrect as it pertains to an increase in size or number, reproduction involves producing offspring, and development refers to the process of growth, maturation, and differentiation of cells and tissues. Therefore, 'Responsiveness' is the most appropriate term for the given description.

Correct Answer: D

Rationale: The correct answer is D: Nucleic acids. Nucleic acids, such as DNA and RNA, are composed of nucleotides and are responsible for storing genetic information and energy (e.g., ATP). Lipids, proteins, and carbohydrates are distinct macromolecules with varying functions and structures. Lipids primarily serve as energy reservoirs, structural components, and cell signaling molecules. Proteins play crucial roles in cellular functions like enzyme activity, structural support, and intracellular signaling. Carbohydrates are mainly utilized as an energy source and for structural purposes within cells.

Correct Answer: B

Rationale: The correct answer is B: Brain. The brain is a vital organ that is part of the central nervous system, along with the spinal cord. It plays a crucial role in processing information, controlling body functions, and coordinating responses to stimuli. The heart, liver, and lungs are not part of the central nervous system; they belong to the cardiovascular and respiratory systems, respectively. The heart is part of the cardiovascular system, responsible for pumping blood throughout the body. The liver is part of the digestive system, involved in processes like detoxification and metabolism. The lungs are part of the respiratory system, facilitating the exchange of oxygen and carbon dioxide during breathing.

Correct Answer: B

Rationale: The correct answer is B. The first heart sound, known as the 'lub' sound, is produced by the closure of the tricuspid and mitral valves, which are the atrioventricular valves. This sound marks the beginning of systole in the cardiac cycle. Choice A, the closing of the aortic and pulmonary valves, is incorrect as these valves produce the second heart sound ('dub'). Choice C, the opening of the aortic and pulmonary valves, is incorrect because the first heart sound occurs during ventricular contraction (systole). Choice D, the opening of the tricuspid and mitral valves, is incorrect as the 'lub' sound is associated with the closure, not the opening, of these valves.