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D. 100 to 1

Henderson-Hasselbalch gives pH = pKa + log([A-]/[HA]). Here pH minus pKa is 2.0, so log of the ratio is 2 and the ratio is 100 to 1 in favor of the conjugate base. The useful shortcut for test day is that every full pH unit above the pKa multiplies the deprotonated fraction by ten. Option A inverts the relationship, which is what happens when you subtract in the wrong direction under time pressure. Option C is the answer you get by treating the two pH unit difference as a factor of two. The MCAT rarely asks you to compute a buffer ratio precisely; it asks whether you know which species dominates and by roughly how much, because that determines charge, solubility and membrane permeability in the biology sections.

B. It quadruples

The continuity equation says A1v1 = A2v2, and area scales with the square of the radius. Halving the radius quarters the cross-sectional area, so the speed must rise by a factor of four to keep the volumetric flow rate constant. Option A is the trap, and it is the single most common error on this question type: candidates apply the radius ratio directly instead of squaring it. Option C reverses the relationship, treating a narrower vessel as slower, which is intuitive from traffic but wrong for an incompressible fluid. Note that this question deliberately says nothing about pressure. Bernoulli would tell you pressure drops where speed rises, and that is a separate step the exam often chains on afterward.

B. It supports the claim by isolating the population as the variable that differed

CARS questions about function are asking what a sentence does for the argument, not whether the sentence is true. If the same policy worked elsewhere, the design cannot be the whole explanation, which pushes the cause back onto the difference between populations, exactly what the author claimed. Option A feels right on a fast read because success elsewhere sounds like evidence against failure, but that reverses the logic. Option D is the strongest distractor, since the statement does imply the design was workable in some context, yet a concession undercuts the argument while this detail strengthens it. On CARS, when two options differ only in whether a detail helps or hurts the thesis, reread the thesis rather than the detail.

B. Standards of judgment shifted rather than the facts about the figure

The word doing the work is "precisely." The same qualities produced praise and then condemnation, so what moved was the standard applied, not the underlying conduct. Option A imports a judgment the sentence carefully avoids; the author is describing a change in reception, not adjudicating merit. Option C invents a claim about historians being ill-informed that nothing in the sentence supports, and CARS punishes that kind of addition heavily. Option D contradicts the sentence, which holds the qualities constant. The discipline this question rewards is refusing to answer a slightly more interesting question than the one asked.

B. A premature stop codon or a frameshift, producing a truncated or nonfunctional protein

Retained intron sequence is translated along with the exons, and intronic sequence is not selected to be in frame or free of stop codons. The overwhelmingly likely result is either a stop codon encountered early or a shifted reading frame downstream. Option A is the trap built directly on a half-remembered fact: introns are non-coding only because they are normally removed, which is exactly the step that failed here. Option D confuses splicing with transcription; splicing happens after and during transcription, so the transcript is still made. The exam repeatedly rewards candidates who can say what happens when a named step in a pathway fails rather than just naming the step.

A. Km increases, Vmax unchanged

A competitive inhibitor binds the active site and competes with substrate, so it can always be outcompeted by piling on more substrate. That is why Vmax is reachable and unchanged, while the apparent Km rises because you need more substrate to hit half maximal velocity. Option B describes noncompetitive inhibition, which is the answer most often swapped in by mistake, and it is worth fixing the logic rather than the pairing: if the inhibitor does not compete for the site, extra substrate cannot rescue it, so Vmax falls. Option D describes what an activator or higher affinity would do. On the MCAT this concept usually arrives inside a passage with a Lineweaver-Burk plot, where competitive inhibition shows a shared y-intercept.

A. Cognitive dissonance reduction through external attribution

The self-concept and the outcome conflict, and the student resolves the discomfort by relocating the cause outside themselves, which is the self-serving bias operating as dissonance reduction. Option B concerns liking things through familiarity and has no bearing here. Option C would require an involuntary response paired with a stimulus, and reorganizing a desk is deliberate. Option D applies to loss of self-awareness in groups. The reason this question type is worth practicing is that MCAT psychology questions rarely ask for a definition; they give behavior and ask which named mechanism it instantiates, which means recognizing the mechanism through an unfamiliar surface story.

A. Social capital and the social determinants of health

Controlling for individual income is the whole point of the design. Once you have removed a person own resources and an effect of place remains, the residual has to be something about the environment: access to care, food availability, air quality, safety, the social networks available. That is the social determinants framework, and neighborhood-level social capital is its usual vocabulary. Option B is a person-perception bias about explaining other people behavior and does not describe an epidemiological result. Option C is contradicted by the design, since neighborhoods are not genetic units. Questions like this reward reading the methods clause, because "after controlling for" is nearly always where the answer is hidden.