Correct Answer: B

Rationale: The correct answer is B: '21%'. Approximately 21% of persons in the United States who have HIV infection are unaware that they are infected. This percentage represents a significant portion of individuals who are not aware of their HIV status, highlighting the importance of increased testing and awareness campaigns. Choices A, C, and D are incorrect as they do not align with the estimated percentage provided in the context.

Correct Answer: A

Rationale: Folate is crucial before conception to prevent neural tube defects. Neural tube development occurs early in pregnancy, often before a woman knows she is pregnant.

Correct Answer: A

Rationale: The correct answer is A: Iron. Iron is an essential nutrient needed by the body to make hemoglobin, the protein in red blood cells that carries oxygen. While other nutrients like vitamin B12, niacin, and folate are important for various bodily functions, they are not directly involved in the production of hemoglobin. Vitamin B12 is essential for nerve function and DNA synthesis, niacin is important for metabolism, and folate is crucial for cell division and DNA synthesis. Therefore, only iron is specifically required for hemoglobin production.

Correct Answer: D

Rationale: The corrected question emphasizes that patients with eating disorders like bulimia should not brush immediately after vomiting as it can worsen enamel erosion due to the acidic content in the mouth. The correct answer is D because patients should rinse with water or a fluoride mouthwash instead of brushing to protect their teeth. Choice A is incorrect because encouraging patients to brush after vomiting is not recommended. Choice B is incorrect as the reason provided is valid but not suitable for the action of encouraging brushing. Choice C is incorrect as the reason for not brushing after vomiting is to prevent enamel erosion.

Correct Answer: C

Rationale: The correct answer is C, Larynx. The larynx is part of the respiratory system, responsible for voice production and breathing. The alimentary canal is involved in the digestive process and includes structures like the oral cavity (mouth), pharynx, and esophagus. Choices A, B, and D are parts of the alimentary canal, where digestion occurs, making them incorrect answers.