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MCAT Sample Questions : Biological Sciences

Passage X

Osteoporosis is a pathological decrease of 35 to 50% in a person's bone mass, which may increase the likelihood of pelvic and wrist fractures and result in back pain or "dowager's hump" due to compression and fracture of the vertebrae.Although all individuals (male and female) steadily lose 0.5 to 1.0% of their bone mass per year after the age of 40, this decline is accelerated in postmenopausal women, who show a loss of 2 to 3% per year for the 8 to 10 years immediately following menopause.They then revert to the slower rate of loss.

Aging-related abnormalities in parathyroid hormone and calcitonin secretion contribute to the slow decline of bone mass in both men and women, as does decreased production of active vitamin D by the kidney.The accelerated postmenopausal bone loss in women appears to be directly linked to estrogen deficiency, although the exact mechanism of this linkage is unknown.

Currently, hormone replacement therapies and dietary calcium supplementation are used in the prevention of osteoporosis in women.Because estrogen therapy may increase the risk of developing uterine and breast cancer, patients on estrogens usually also receive low doses of progesterone analogues, which appear to substantially diminish this risk.Recently, calcitonin analogues have also been developed for the treatment and prevention of osteoporosis.

Following are some sample questions on this passage:

  1. In addition to the effects of estrogen deficiency, the most likely reason that more women than men suffer from osteoporosis is that women, compared to men, have:

    1. lower bone density.
    2. fewer vertebrae.
    3. less efficient mechanisms for calcium uptake.
    4. less efficient vitamin D production.

    Answer: A

    Explanation: The correct answer is answer choice A: that women have lower bone density than men.Men and women have the same number of vertebrae, and there is no evidence presented in the passage to suggest that there are sex differences in calcium uptake or vitamin D production.Lower initial bone density would increase the impact of any subsequent bone loss.

  2. Postmenopausal women receiving estrogen and progesterone therapy will most likely experience which of the following side effects?

    1. Breast tissue will atrophy.
    2. Vaginal tissue will dry out.
    3. Periodic menstruation will resume.
    4. Lactation will be induced.

    Answer: C

    Explanation: Estrogen and progesterone are actively secreted by the ovaries of pre-menopausal women and act to maintain the uterine cycle.With advancing age the ovary becomes less responsive to pituitary gonadotropins and cyclical changes in the endometrium of the uterus disappear.The menstrual cycle can be re-established by administration of estrogen and progesterone in a regimen that approximates the rise and fall of hormone levels in pre-menopausal women.So the correct answer is answer choice C, periodic menstruation will resume.To the extent that the therapeutic dosages are typical of pre-menopausal women, side effects such as breast tissue atrophy (answer A), vaginal tissue drying (answer B) and lactation (answer D) would not be expected to occur.

  3. Production of which of the following hormones will be inhibited by the administration of dietary calcium to prevent osteoporosis?

    1. Growth hormone
    2. Calcitonin
    3. Thyroid hormone
    4. Parathyroid hormone

    Answer: D

    Explanation: Calcium levels in the blood need to be kept constant.Parathyroid hormone and calcitonin regulate blood levels of calcium.The passage gave the information that calcitonin analogs inhibit osteoporosis.If calcitonin inhibits osteoporosis, it must function to take calcium out of the blood and into the bone while preventing the loss of calcium from bone into the blood.High levels of blood calcium available for deposit into bone should stimulate this process rather than inhibit it.Thus answer choice B, calcitonin, must be incorrect.Because growth hormone (answer A) and thyroid hormone (answer C) are not involved in the regulation of blood calcium levels, they are also incorrect.The correct answer must be parathyroid hormone (answer D) which is inhibited by high levels of calcium.A mnemonic students use to remember which hormone puts calcium into bone and which gets rid of it is: Calcitonin-in, parathroid–rid.

  4. A man is treated with low doses of an estrogen analogue to destroy an estrogen-responsive adrenal tumor.Compared to an age-matched control (no estrogen treatment), this patient's chances of developing osteoporosis will most likely be:

    1. increased.
    2. decreased.
    3. approximately the same.
    4. approximately the same, but the disease will appear at an earlier age.

    Answer: C

    Explanation: The passage only states that post-menopausal women show the accelerated bone loss and the acceleration slows after eight to ten years.From this it seems likely that the acceleration has something to do with withdrawal of estrogen.Administration of estrogen to men would then have no therapeutic effect on bone loss, since they should not be suffering from withdrawal of the hormone, having never had high levels.Men given estrogen should have the same chance of developing osteoporosis as a control population, answer choice C.

  5. Osteoblasts, which form bone, and osteoclasts, which resorb it, work together to cause continuous bone remodeling.In a person suffering from osteoporosis, which of the following combinations of changes in the activity of these cell types will most likely occur?

    1. Increased osteoblast and decreased osteoclast activity
    2. Increased osteoblast and increased osteoclast activity
    3. Decreased osteoblast and decreased osteoclast activity
    4. Decreased osteoblast and increased osteoclast activity

    Answer: D

    Explanation: Osteoporosis involves two factors, decreased formation and increased breakdown of bone.Osteoblasts mediate the former process and osteoclasts the latter.Decreased osteoblast activity and increased osteoclast activity result in bone loss, answer choice D.

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