MCAT Question of the Day

Parathyroid hormone (PTH) is a peptide hormone produced in the parathyroid gland. It plays a major role in increasing blood calcium (Ca) levels, primarily through a negative feedback loop. Which of the following statements best describes how this feedback loop works?

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Multicellular organisms are always composed of many small cells rather than fewer large cells. As a cell’s surface area increases by the square of its linear dimension, its volume increases by the cube of its linear dimension. Which statement is true about how cellular functions would be directly affected in a cell that grew too large?

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If a cell is treated with a substance that inhibits DNA polymerase activity, what stage of the cell cycle is most likely to be affected?

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Today’s question is submitted by Angela Boyd of Westminster College.

Which of the following gives the relative amounts of the smooth and rough endoplasmic reticulum in liver vs. pancreatic cells?

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Blood pressure is a function of cardiac output (heart rate x stroke volume), as well as peripheral resistance (a measure of friction between the blood and the blood vessel walls). Factors that can increase peripheral resistance include:

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Rigor mortis, a condition in which skeletal muscles temporarily remain in a rigid state of partial contraction due to the absence of ATP, occurs a few hours after death. Which of the following steps is NOT necessary for muscle relaxation?

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Correct Answer: C. The detachment of the troponin-tropomyosin complex from actin.

Choices A, B, and D are all necessary for a muscle cell to return to its resting state. The troponin-tropomyosin complex must return to its original position on actin (the position it occupied before calcium was released from the sarcoplasmic reticulum). When the complex detaches, muscle contraction occurs.

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Individuals with Turner’s Syndrome have 45 chromosomes (one sex chromosome is missing in the cells of these “XO” individuals). This usually is caused by meiotic nondisjunction during gametogenesis. Which statement is NOT correct concerning people with this disorder?

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Correct Answer: B. They are subject to sex-linked disorders at frequencies similar to those predicted for normal females.

Since normal females have two X-chromosomes, Turner’s Syndrome females do not have the same probabilities of inheriting sex-linked disorders (they actually have the same chances as males).

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If a muscle cell is stimulated a second time before it has time to relax completely, a stronger contraction results. What is this phenomenon called?

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Curare is a chemical used in arrow poisons as well as in pharmaceutical compounds. It competes with acetylcholine for receptor sites along the motor end plate. The immediate physiological effects of high doses of curare can include:

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During respiratory ventilation, impulses along the phrenic nerves stimulate the diaphragm, causing it to contract and move downward. When this occurs, which of the following takes place in the thoracic cavity and lungs?

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