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Solutions to 100 passage-based Biology questions

Solutions to 100 passage-based Biology questions - Prep101...

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© Prep101 http://www.prep101.com/mcat/ Page 1 of 12 1. A. Calcitonin reduces bone resorption. Bone resorption occurs when the level of calcium in the blood plasma is low, but resorption is not needed when the level of calcium is high. Therefore, resorption would be reduced by calcitonin under conditions in which the level of calcium in the plasma is high. 2. B. The passage states that vitamin D is nonpolar and that it can be obtained through the action of ultraviolet rays on the skin. Nonpolar molecules tend to be lipid-soluble rather than water-soluble, and exposure to ultraviolet rays tends to be less in northern climates than in tropical climates. Of the choices given, a child who has a dietary deficiency of fat-soluble vitamins and lives in a northern climate would be most likely to develop rickets. 3. D. Activated vitamin D acts on the small intestine to stimulate the absorption of calcium into the bloodstream. The inclusion of vitamin D in calcium supplements would ensure that vitamin D is present in the body to help promote this absorption. 4. D. The passage states that rickets is caused by insufficient vitamin D activity. Insufficient vitamin D activity would reduce the ability of the body to absorb ingested calcium from the small intestine. To maintain calcium levels in the blood plasma, parathyroid hormone would promote the breakdown of bone tissue, causing the bones to become weak. If there were a metabolic deficiency of parathyroid hormone, the body would be unable to break down bone tissue (option I), causing a higher than normal ratio of mineral to organic matter in the bones instead of a lower than normal ratio. However, if the body were unable to convert vitamin D to its active form, or if vitamin D were unable to act on its target tissue, overall vitamin D activity would be impaired, which can lead to rickets. 5. C. When the level of calcium in the blood plasma is low, the body responds by mobilizing stores of calcium from the bones via the activity of parathyroid hormone. Parathyroid hormone will increase the number of osteoclasts, which break down bone cells. Therefore, one would expect an increase in both parathyroid hormone and osteoclast activity in order to increase the level of calcium in the blood plasma. However, vitamin C promotes bone formation, a process that would further lower the calcium level in the plasma. 6. D. Removal of the parathyroid gland would lead to hypocalcemia , a condition of low blood calcium, resulting from the lack of parathyroid hormone. This would cause increased neuromuscular excitability because of the change in membrane potential, which under normal physiological conditions, is partially kept in balance with extracellular calcium. Typically, the person would eventually die from severe respiratory muscle spasms.
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