MCAT Question of the Day

Correct Answer: A. Gastrula

The correct answer is A. The oocyte and morula are both precursors to the blastula stage. The embryoblast and the trophoblast are both components of the blastula, but are not the next stage in embryonic development. Once the blastula undergoes gastrulation, it will become a gastrula with three distinct layers of tissue: endoderm, mesoderm, and ectoderm.

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Correct Answer: A. Irreversible competition.

Penicillin covalently binds to the active site of the bacterial enzyme and permanently disables its action. This mode of action is inherent to irreversible inhibitors. Notice this is similar to competitive inhibitors, but competitive inhibitors are reversible and adding more substrate can overcome the competitive inhibitor. Here is an excellent explanation on enzyme mechanics.

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Correct Answer: D. losing an existing allele due to random sampling.

Genetic drift represents the change in an allele frequency in a population due to random sampling. Influence from genetic drift is particularly important in small populations. The random changes in allele frequencies could result in the elimination of an existing allele from a population.

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Correct Answer: B. tert-butyl carbocation

The most stable fragment likely formed from 2,2-dimethylpentane fragmentation is t-butyl species.

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Correct Answer: C. N2(g) 0.75 and H2(g) 2.25 mol.L^-1.s^-1

The rate of consumption of the reactants will be the product of the rate of formation of the product multiplied by the ratio of the (co-efficient of the reactant) / (co-efficient of the product).

Rate N2(g) = 1.50 x 1/2 = 0.75 mole.L^-1.s^-1 and H2(g) 1.50 x 3/2 = 2.25 mole.L^-1.s^-1.

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Correct Answer: D. Benzoic acid, an organic compound, is insoluble in water. Extraction into water can be achieved with aqueous sodium hydroxide (deprotonation -> polar sodium benzoate, now soluble in water). Transfer of the benzoate back into organic solution is accomplished by treatment with hydrochloric acid (protonation -> less polar benzoic acid).

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Correct Answer: A. Speed = 6.67 m/s and Velocity = zero

Speed is a scalar quantity involving magnitude only: Speed = distance/time = 400/60 = 6.67 m/s. Velocity is a vector quantity involving both magnitude and direction: Velocity = displacement/time = 0/60 = zero.

Distance traveled is not dependent on a direction, but displacement is. Displacement = distance from the finish to the start, which in this case is zero as the athlete has run around an oval to arrive back at her starting place.

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Correct Answer: C. cellular respiration

In the cell, metabolism is defined as the chemical reactions that occur within the cell to sustain life. Metabolism can further be divided into anabolism (the synthesis of new compounds) and catabolism (breaking down of substances). Answer options A, B, D, and E are all anabolic processes, that require the use of energy to make new compounds. In cellular respiration, glucose is broken down to produce ATP, therefore this is a catabolic process.

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Correct Answer: B. 1/16

F1 generation is the heterozygous offspring of the homozygous parents. F2 generation is the offspring of the heterozygous offspring generation F1. The ratio is 9:3, 3:1 in this generation, meaning that only one of the offspring will have black fur and a long tail.

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Correct Answer: B. Equilibrium favors the products.

In an acid-base equilibrium such as this, the relative strength of an acid can be determined using the magnitude of Ka (or pKa): as Ka increases, the strength of the acid increases; as pKa (inverse log of Ka) decreases, the strength of the acid increases. The equilibrium lies to the left because t-butanol is the weaker acid compared to water. You can also think this way: hydroxide is the weaker base (more stable) compared to t-butoxide because of e- donation of alkyl groups, making the resultant anion less stable (more reactive).

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