Which is absorbed mainly in the colon?

Correct Answer: A

Rationale: The correct answer is A: short chain fatty acids. Short chain fatty acids are primarily absorbed in the colon through passive diffusion. The colon plays a crucial role in absorbing these fatty acids due to its high concentration of beneficial gut bacteria that break down dietary fiber into short chain fatty acids. Calcium is mainly absorbed in the small intestine, vitamin K is absorbed in the small intestine and partially in the colon, and oligosaccharides are mainly broken down by enzymes in the small intestine.

Correct Answer: C

Rationale: The correct answer is C because the substrates of the flavoprotein-cytochrome system are pyruvate, water, oxygen, and ATP. Pyruvate is oxidized to produce energy in the form of ATP, water is a byproduct of the electron transport chain, and oxygen is the final electron acceptor. Choices A and B are incorrect because cytochrome oxidase is not the first step in the chain, and the system does not occur within the endoplasmic reticulum. Choice D is incomplete.

Correct Answer: C

Rationale: The correct answer is C because steroid hormones are lipid-soluble and can diffuse through cell membranes. Once inside the cell, they bind to specific receptors in the cytoplasm. This hormone-receptor complex then enters the nucleus to regulate gene expression. Option A is incorrect because steroid hormone receptors are not found on nutrient molecules like glucose. Option B is incorrect as these receptors are not typically located in the interstitial fluid. Option D is also incorrect because steroid hormone receptors are not located on the cell membranes of target tissues.

Correct Answer: B

Rationale: Step-by-step rationale for why choice B (thyroid hormone) is correct: 1. Thyroid hormone is a key hormone that regulates metabolism and growth. 2. Intracellular hormone receptors are specific to certain hormones. 3. Disruption of thyroid hormone levels can lead to metabolic disorders. 4. Pesticide binding to the thyroid hormone receptor can interfere with its normal function. Summary of why other choices are incorrect: A: Melatonin regulates sleep-wake cycles, not metabolism. C: Growth hormone regulates growth and development, not metabolism. D: Insulin regulates blood sugar levels, not metabolism.

Correct Answer: A

Rationale: The correct answer is A: Closes trachea when swallowing. The epiglottis is a flap of tissue that covers the trachea (windpipe) during swallowing to prevent food or liquid from entering the airway. It ensures that food goes down the esophagus instead of the trachea. This action helps protect the respiratory system from aspiration. Choices B, C, and D are incorrect. B is incorrect because the epiglottis closes the trachea, not the esophagus, during swallowing. C is incorrect because the trachea is supported by rings of cartilage, not the epiglottis. D is incorrect because the epiglottis does not close the nasal cavity but rather the trachea.