A two-week-old boy develops persistent projectile vomiting. The most likely diagnosis is:

Correct Answer: A

Rationale: The correct answer is A: pyloric stenosis. In pyloric stenosis, there is hypertrophy of the pyloric sphincter muscle, leading to gastric outlet obstruction and projectile vomiting. This typically presents in infants around 2-6 weeks of age. Other choices (B) esophageal atresia presents with difficulty feeding and choking, (C) annular pancreas with duodenal obstruction, and (D) incomplete rotation of the gut with volvulus or obstruction due to malrotation. Pyloric stenosis is the most likely diagnosis in this scenario based on the age of the infant and the symptom of projectile vomiting.

Correct Answer: B

Rationale: The correct answer is B: Sleep. During sleep, the body's metabolic rate decreases as it enters a state of rest and recovery. Pregnancy (A) requires increased energy expenditure for fetal development. Males generally have higher metabolic rates than females (C) due to higher muscle mass. Walking (D) requires more energy than resting, leading to a higher metabolic rate. In summary, sleep is the exception as it lowers metabolic rate compared to the other activities listed.

Correct Answer: C

Rationale: The correct answer is C: Starch. Amylase is an enzyme that breaks down starch into maltose in the oral cavity. This process is the initial step in carbohydrate digestion. Proteins are primarily digested in the stomach by pepsin. Lipids are broken down by lipase in the small intestine. Amino acids are the end product of protein digestion, not the substrate for amylase. Therefore, the correct answer is C as amylase specifically targets starch for digestion.

Correct Answer: D

Rationale: Pepsin is the correct answer as it is the enzyme specifically produced by gastric chief cells in the stomach to break down proteins into peptides. It works optimally in the acidic environment of the stomach. Amylase (choice A) breaks down carbohydrates, protease (choice B) refers to a general term for enzymes that break down proteins, and lipase (choice C) breaks down fats, making them incorrect for this question.

Correct Answer: A

Rationale: The correct answer is A: mouth. Salivary amylase, an enzyme in saliva, initiates the breakdown of starch into maltose in the mouth. This process marks the beginning of chemical digestion of starch foods. In contrast, the stomach primarily digests proteins, the small intestine further breaks down carbohydrates, proteins, and fats, while the large intestine absorbs water and minerals. Therefore, options B, C, and D are incorrect as they do not represent the initial site of starch digestion.