Correct Answer: C

Rationale: The correct answer is C: Ischemia. In this scenario, the 75-year-old male experiences chest pain on exertion, which is indicative of angina. Angina is primarily caused by reduced blood flow to the heart muscle, leading to hypoxic injury. This condition is known as ischemia. Options A, B, and D are incorrect. Malnutrition does not typically cause chest pain related to exertion. Free radicals and chemical toxicity are not common causes of chest pain in the context described. Therefore, the most likely cause of chest pain in this case is ischemia due to reduced blood flow.

Correct Answer: D

Rationale: The correct answer is D: Use of nitrates. Sildenafil (Viagra) is contraindicated in patients taking nitrates due to the risk of severe hypotension. Nitrates and sildenafil both cause vasodilation, and the combination can lead to a dangerous drop in blood pressure. Choice A, peptic ulcer disease, is not a contraindication for sildenafil use. Choice B, a history of myocardial infarction, is not a contraindication unless the patient is also taking nitrates. Choice C, recent use of antihypertensive medications, is not a contraindication but requires caution as the combination can lead to additive effects lowering blood pressure.

Correct Answer: B

Rationale: The correct answer is B: 'Use of nitrates.' Tadalafil (Cialis) is contraindicated in patients taking nitrates due to the risk of severe hypotension. Nitrates and Cialis both cause vasodilation, which can lead to a dangerous drop in blood pressure when used together. Choices A, C, and D are incorrect because a history of hypertension, use of antihypertensive medications, and a history of peptic ulcer disease are not specific contraindications for tadalafil use.

Correct Answer: B

Rationale: The correct answer is B: Rh antigen. In addition to ABO antigens, Rh antigen must also be matched for a blood transfusion. Rh antigen compatibility is crucial to prevent adverse reactions. Choice A, HLA type, is not directly related to blood transfusions but plays a role in organ transplantation. Choice C, Immunoglobulins, are not typically matched for blood transfusions. Choice D, Platelet compatibility, while important in specific cases, is not a standard requirement for all blood transfusions.

Correct Answer: B

Rationale: The correct answer is B. Immunization involves administering an antigen, such as a weakened or killed microorganism, to stimulate the immune system to produce an antibody response. This process helps the body recognize and remember specific pathogens, providing immunity against future infections. Choice A is incorrect because immunizations are administered to the infant directly, not to the pregnant woman before birth. Choice C is incorrect as there is no scientific evidence linking immunizations to autism. Choice D is incorrect as immunizations protect against infectious agents, not antibodies.