Examkin

Question

Difficulty: Very hardWeaken

Researcher: The hormone somatotropin stimulates growth in juvenile *Rana clamitans*. In a strictly controlled laboratory study, two groups of these frogs were observed for six months. Both groups were provided with identical diets and environmental conditions, differing only in the water's dissolved calcium concentration. The frogs in the high-calcium water exhibited growth rates 30 percent higher than those in the low-calcium water. Because both groups of frogs synthesized identical amounts of somatotropin over the study period, the dissolved calcium must directly enhance the sensitivity of the frogs' cellular growth receptors to this hormone.

Which one of the following, if true, most seriously weakens the researcher's argument?

  1. A
    In *Rana clamitans*, cellular growth receptors undergo a structural alteration in the presence of calcium that allows somatotropin molecules to bind more efficiently.
  2. B
    Further analysis of the frogs in the high-calcium group revealed that they actually synthesized slightly more somatotropin than the frogs in the low-calcium group.
  3. C
    Juvenile *Rana clamitans* in the wild typically experience their most rapid growth spurts during seasonal heavy rains, a period when pond calcium concentrations are heavily diluted.
  4. High concentrations of dissolved calcium significantly inhibit the enzymes responsible for the natural breakdown of somatotropin in the bloodstream of *Rana clamitans*.Answer
  5. E
    In many amphibian species, artificially inducing rapid growth naturally causes the animal to absorb and retain higher concentrations of dissolved calcium from its environment.

Answer

The statement indicating that high concentrations of dissolved calcium inhibit the enzymes responsible for the natural breakdown of somatotropin.
The argument concludes that calcium must make cellular receptors more sensitive to somatotropin because both groups synthesized identical amounts of the hormone but the high-calcium group grew more. However, this assumes that synthesis rate is the only factor determining how much hormone is available to the body. The correct answer introduces a crucial alternative mechanism: calcium prevents the biological breakdown of the hormone. If somatotropin degrades more slowly, its net concentration in the bloodstream remains higher over time. This higher sustained concentration perfectly explains the increased growth without requiring any change in receptor sensitivity, thereby destroying the argument's core assumption.

Step-by-Step Solution

1
Analyze the argument's structure to identify the premises and conclusion.
Premises: High calcium leads to higher growth. Both groups synthesized identical amounts of somatotropin. Conclusion: Calcium directly enhances receptor sensitivity.
Understanding the logical core is necessary before evaluating structural weaknesses.
2
Identify the logical gap or assumption in the researcher's reasoning.
The researcher assumes that if the synthesis (production) amounts are identical, the only way for the hormone to have a greater effect is if the cellular receptors become more sensitive.
Identifying the unstated assumption reveals the argument's primary vulnerability.
3
Formulate a potential weakener based on the logical gap.
An effective weakener would provide an alternative reason why identical synthesis could lead to higher growth, such as the hormone lasting longer before degrading.
Pre-phrasing an answer helps accurately evaluate and eliminate the given options.
4
Evaluate the given options against the pre-phrased weakener.
The option regarding the inhibition of breakdown enzymes perfectly matches the alternative mechanism.
This confirms the correct option while ensuring distractors are dismissed logically.

Key Concept

Identifying Alternate Explanations and Unstated Assumptions
Rate this question

Topics

Weaken

More questions from this topic