Practice Test 6 - MCAT Biological and Biochemical Foundations of Living Systems

20 questions. Answer them in any order, then submit to see your score and every explanation.

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1. Investigators tested a cytosolic protein (Protein L) that binds phosphoinositide lipids at the inner leaflet of the plasma membrane. A mutant lacking the lipid-binding domain remained cytosolic. Cells expressing the mutant showed impaired recruitment of an actin-regulatory complex to the membrane during chemotaxis, though the complex's abundance was unchanged. Based on the scenario, which outcome is most consistent with Protein L's function?
2. A lab observes that gene RPLP0 mRNA levels are unchanged across conditions, but RPLP0 protein levels increase after stress Q. Polysome profiling shows more RPLP0 mRNA in heavy polysome fractions after Q. Which statement is most consistent with these findings in the context of eukaryotic gene regulation?
3. A 9-month-old has failure to thrive, chronic diarrhea, and metabolic acidosis. Plasma shows elevated propionyl-CoA-derived metabolites and increased anion gap. Enzyme assay reveals reduced propionyl-CoA carboxylase activity. Based on the biochemical concept of anaplerotic entry into the TCA cycle, which consequence is most consistent?
4. In a skeletal muscle fiber, a brief depolarizing current pulse normally triggers an action potential. A peptide is applied that selectively increases K$^+$ conductance during the early depolarization but does not alter Na$^+$ channel availability. Intracellular/extracellular concentrations (mM) are K$^+$ 140/4 and Na$^+$ 12/145. Based on the scenario, what effect would increasing K$^+$ conductance have on the likelihood of reaching threshold for an action potential?
5. In epithelial cells, a fluorescent glucose analog was taken up rapidly under baseline conditions. When extracellular Na$^+$ was replaced with an impermeant cation, uptake decreased markedly, but the plasma membrane expression of the glucose transporter (Transporter G) was unchanged. Based on the scenario, which outcome is most consistent with the transporter's function?
6. In a skeletal muscle fiber, a reagent selectively oxidizes ryanodine receptors, increasing their open probability at rest. Resting cytosolic Ca$^{2+}$ rises from 0.10 $\mu$M to 0.35 $\mu$M, while SR Ca$^{2+}$ content gradually decreases. The muscle shows mild resting tension and reduced peak twitch force over time. Which statement best describes the role of calcium in this process?
7. Researchers evaluated luteal function after administration of an aromatase inhibitor during the late follicular phase (days 9–13). Compared with placebo, treated participants had estradiol 55 pg/mL (vs 170 pg/mL) on day 13, with LH 12 IU/L (vs 7 IU/L) and FSH 10 IU/L (vs 6 IU/L). Progesterone measured 7 days later was 6.5 ng/mL (vs 10.2 ng/mL). Which hormonal change would be expected under these conditions?
8. In a controlled heat exposure experiment (ambient 38°C), participants wore a water-impermeable suit that prevented sweat from evaporating but allowed sweat to be produced. Skin conductance increased (consistent with sweating), but core temperature rose faster than in controls wearing breathable clothing. The thermoregulatory principle is evaporative cooling via eccrine sweat. Which outcome is most consistent with thermoregulation in this scenario?
9. In a neuromuscular junction model, a drug was applied that selectively inhibits presynaptic voltage-gated Ca$^{2+}$ channels without affecting axonal Na$^+$ channels. The concept tested is neurotransmitter release at chemical synapses. When the motor neuron is stimulated with the same action potential train, which change is most expected at the muscle end plate? Miniature end-plate potential (mEPP) amplitude is unchanged by the drug.
10. In cardiomyocytes, investigators examined how a cytosolic Ca$^{2+}$-binding protein (Protein B) shapes calcium transients. Protein B binds Ca$^{2+}$ reversibly and does not catalyze chemical reactions. Overexpression of Protein B reduced the peak amplitude of free cytosolic Ca$^{2+}$ during stimulation but prolonged the decay back to baseline. Which outcome is most consistent with Protein B's function?
11. A double-stranded DNA bacteriophage is used for generalized transduction. After infection of donor bacteria, some released phage particles package fragments of host bacterial DNA instead of phage DNA. These particles can deliver donor genes to recipient bacteria but do not produce plaques unless co-infected with wild-type phage. Which statement best reflects the replication strategy illustrated?
12. In an experiment, participants inhale a gas mixture that increases arterial $\text{CO}_2$ from 40 mmHg (baseline) to 50 mmHg for 5 minutes. Ventilation rate rises and arterial pH moves back toward baseline. Based on the scenario, which outcome aligns with the feedback loop regulating acid-base status?
13. In an experiment on anaphase movement, cells were briefly exposed to a drug that selectively slows depolymerization at kinetochore microtubule plus-ends while leaving spindle midzone microtubules largely unaffected. After anaphase onset, sister chromatids moved poleward more slowly than controls, but spindle poles still separated at near-normal rates. Total anaphase duration increased. Based on the study, which conclusion about the forces driving chromosome movement is most consistent with the data?
14. A bird species has two beak shapes determined largely by a single locus: narrow (N) and broad (B). During a period when only large, hard seeds are available, broad-beaked birds have higher feeding efficiency and produce more fledglings. Allele B rises from 0.25 to 0.55. Which statement best reflects the process of natural selection described?
15. A cell-free cytosolic extract from human erythrocytes was supplied with glucose-6-phosphate (G6P) as the only carbon input. Under condition 1, NADP$^+$ was abundant; under condition 2, NADPH was added to raise the NADPH/NADP$^+$ ratio while keeping total NADP(H) constant. All other substrates and cofactors were unchanged. The investigators monitored CO$_2$ release from the oxidative branch of the pentose phosphate pathway (PPP) over 5 minutes. Based on the scenario, which metabolic shift would be expected when the NADPH/NADP$^+$ ratio is increased?
16. A 35-year-old with an insulin-secreting pancreatic tumor has recurrent fasting hypoglycemia. During an episode, labs show: glucose 2.2 mM, insulin high, C-peptide high, and low plasma free fatty acids. The clinician explains that one reason symptoms occur rapidly is reduced availability of alternative fuels for the brain. Which process is most likely suppressed by the patient's hormonal state?
17. In a diploid frog species, a specific autosome carries a visible cytological inversion in one homolog (an inversion heterozygote). Researchers cross an inversion heterozygote to a normal homozygote and score offspring viability. They observe a substantial fraction of embryos that arrest early, while surviving offspring inherit either the normal arrangement or the inverted arrangement without obvious recombination within the inverted segment. Which explanation best accounts for reduced viability and suppressed recovery of recombinants within the inversion?
18. In a renal physiology study, investigators infused angiotensin II at a dose that preferentially constricted efferent arterioles. Renal blood flow decreased, but glomerular filtration rate (GFR) was maintained near baseline for several minutes. The central principle is that altering resistance at specific vascular segments changes local pressures and flows to support homeostasis of filtration. Which conclusion is most consistent with these findings?
19. In an experiment focused on conjugation, a donor strain is engineered to be F− but still carries a non-conjugative plasmid encoding resistance. When mixed with an F− recipient, no transfer is observed. Which statement is most consistent with the outcome?
20. A virus with a dsRNA genome infects intestinal epithelial cells. Early after infection, viral mRNAs are detected despite host transcription inhibition, and virions contain an internal enzyme activity that synthesizes RNA from RNA templates. Based on the information, which conclusion is most consistent with the viral life cycle?

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