Correct Answer: C

Rationale: The correct answer is C: bicarbonate ions. Bicarbonate ions, secreted by the pancreas, neutralize the acidic chyme entering the small intestine from the stomach, creating a more suitable environment for digestive enzymes. Saliva (choice A) helps in the initial breakdown of food in the mouth, not in neutralizing stomach acid. Gastric mucus (choice B) protects the stomach lining from the acidic environment but does not neutralize the acid in the small intestine. Enzymes (choice D) facilitate chemical reactions in digestion but do not neutralize stomach acid.

Correct Answer: B

Rationale: AIDS enteropathy typically presents as diarrhea and weight loss due to the impact of HIV on the gastrointestinal tract. While abdominal pain and rectal bleeding (Choice A), abdominal bloating and flatulence (Choice C), and rectal fissures and constipation (Choice D) can occur in some cases, the most common manifestations are diarrhea and weight loss.

Correct Answer: A

Rationale: Excessive supplementation of calcium and vitamin D beyond the upper intake level is not recommended unless under medical supervision, as it can cause adverse health effects.

Correct Answer: B

Rationale: Alcohol does contain energy and provides calories, but it is not considered a nutrient. Nutrients are substances that contribute to essential bodily functions. While alcohol may have energy, it does not provide essential vitamins, minerals, or other nutrients necessary for the body's health. Additionally, excessive alcohol consumption can have harmful effects on the body, further supporting why alcohol is not classified as a nutrient.

Correct Answer: A

Rationale: The statement is TRUE. Sickle cell disease is caused by a genetic mutation in the hemoglobin gene, leading to an abnormal amino acid sequence. This results in the production of abnormal hemoglobin molecules, causing red blood cells to become sickle-shaped. This inherited condition is a classic example of a genetic error affecting the amino acid sequence, making choice A the correct answer. Choices B, C, and D are incorrect as they do not accurately reflect the nature of sickle cell disease.