Practice Test 10 - 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. In a pH 7.4 buffer, a purified enzyme is briefly exposed to 6 M urea and then allowed to refold. The enzyme regains activity only when a cluster of hydrophobic side chains can repack into the protein core. Which amino acid substitution at a buried position is most likely to disrupt refolding by introducing a strongly polar side chain while keeping the backbone (NH$_3^+$/COO$^-$) unchanged?
2. A freshwater fish species has two color morphs controlled by alleles G (green) and S (silver). In a clear lake with abundant aquatic plants, birds preferentially catch silver fish; in a nearby murky lake with little vegetation, birds preferentially catch green fish. A researcher transplants 200 fish from each lake into large enclosures in the other lake (no migration afterward). After 5 generations, allele frequencies shift toward the locally favored morph in each enclosure. Which statement best reflects the process of natural selection described?
3. A bacteriophage lysate is used to transfer a metabolic gene from a donor bacterium to a recipient. The recipient acquires the new metabolic capability only when the donor and recipient share substantial sequence homology around the transferred gene. When the donor gene is from a distantly related species with low homology, the new trait is rarely observed, despite similar infection rates. Which statement best describes the process of viral transduction in the scenario?
4. In a large population of insects, a pesticide is applied annually. A resistance allele R exists at low frequency before application. After several years, R increases substantially. Resistance is costly in pesticide-free lab conditions. Which statement best reflects the process of natural selection described?
5. A developmental biologist tracks a germ cell lineage and notes that one cell completes meiosis I and then pauses before finishing meiosis II. The cell has separated homologous chromosomes but retains sister chromatids. When fertilization occurs, meiosis II completes and the second polar body is extruded. Which outcome would be expected from the described meiotic event?
6. A physiology lab compares two endocrine signals: Hormone X is synthesized from cholesterol in the adrenal cortex and circulates largely bound to plasma proteins; Hormone Y is synthesized as a preprohormone in the anterior pituitary and circulates freely in plasma. After acute secretion, target cells respond to X with changes in gene expression that peak after several hours, whereas target cells respond to Y within minutes by altering phosphorylation states of existing enzymes. Based on the vignette, which statement best explains the mechanism of action for Hormone X? (Assume both hormones participate in negative feedback at the hypothalamus/pituitary axis.)
7. Two subviral particles are studied for how they persist without encoding polymerases. Particle A is associated with neurodegeneration; its activity is resistant to nucleases and correlates with conversion of a host protein to a β-sheet–rich, protease-resistant form. Particle B is associated with plant stunting; it is a small circular RNA found in the nucleus and produces abundant 21–24 nt small RNAs. Which statement most accurately compares how A and B increase their effective copy number in host cells?
8. A study of chromosome alignment used cells with a mild reduction in kinetochore-microtubule attachment strength. At metaphase, most chromosomes appeared aligned, but tension-sensitive reporters at several kinetochores indicated low tension despite apparent alignment. Shortly after, missegregation events increased. Based on the study, which conclusion about metaphase alignment is most consistent with the data?
9. In a vertebrate embryo culture, investigators label epiblast cells at the onset of gastrulation and track their positions 6 hours later. Labeled cells that ingress through the primitive streak disperse between the outer epithelial sheet and the inner lining of the yolk sac, forming a loosely organized middle layer that later segments into somites. Tight junctions are maintained in the outer sheet, while the inner lining becomes a continuous epithelial tube-like layer. Based on these observations, which embryonic layer is most likely to develop into axial and paraxial skeletal muscle in this system?
10. A patient with chronic obstructive pulmonary disease has baseline arterial $P_{CO_2}$ of 55 mmHg with near-normal pH due to renal compensation. When given a high-flow oxygen mask, ventilation decreases and $P_{CO_2}$ rises further. Based on the scenario, which outcome aligns with the feedback loop controlling ventilation in this patient?
11. During fasting, a volunteer's plasma Ca2+ drifts slightly below baseline. The skeleton can restore plasma Ca2+ by increasing osteoclast-mediated bone resorption (releasing Ca2+ and phosphate), while osteoblasts deposit mineral during formation. PTH increases in response to low Ca2+, shifting remodeling toward resorption; calcitonin has the opposite effect. Which change would most directly help restore plasma Ca2+ in this setting?
12. A postsynaptic neuron is voltage-clamped at $-70$ mV. Activation of a ligand-gated cation channel produces an inward current. When the membrane is clamped at 0 mV, activation of the same channel produces ~0 net current. The channel is permeable to Na$^+$ and K$^+$. Which mechanism best explains the near-zero current at 0 mV?
13. A study compared arterial oxygenation in patients with severe emphysema versus matched controls. Imaging showed destruction of alveolar walls in emphysema. Which observation best supports that the primary gas exchange limitation is reduced surface area rather than reduced ventilation?
14. A 2-day-old infant has seizures and apnea. Plasma glycine is markedly elevated; CSF glycine is also elevated with an increased CSF:plasma glycine ratio. Sequencing shows a mutation in the P protein of the glycine cleavage system that reduces glycine decarboxylation. The central biochemical concept is accumulation of a neurotransmitter due to impaired catabolism. Which outcome is most consistent with the mutation?
15. During purification, a peptide is placed in strongly acidic buffer (pH 2). Consider a residue with side chain $R = \text{CH}_2\text{-COO}^-$. Which change in the side chain's protonation state is most consistent with this condition?
16. A lab tests whether an iPSC preparation remains pluripotent after repeated passaging. Two aliquots are taken from the same line: Aliquot 1 is analyzed immediately; Aliquot 2 is cultured for 3 additional weeks under conditions intended to maintain an undifferentiated state. When exposed to three different differentiation media (neural, hepatic, cardiac), Aliquot 1 generates all three lineages, while Aliquot 2 generates only neural cells and shows poor survival in hepatic and cardiac media. Which conclusion is most consistent with reasoning about pluripotency from these observations?
17. A patient with a traumatic transection of the pituitary stalk is evaluated 2 weeks after injury. Basal serum cortisol is low, and ACTH is low. Serum prolactin is elevated. MRI shows an intact anterior pituitary but loss of portal blood flow from the hypothalamus. Which pattern is most consistent with disruption of hypothalamic neural regulation of anterior pituitary endocrine function?
18. A subject begins a 36-hour fast. At hour 6, insulin is low and glucagon is elevated; at hour 30, insulin remains low and glucagon remains elevated, but respiratory quotient decreases. The investigator interprets the later time point as increased reliance on lipid-derived fuels. Which change is most likely at hour 30 compared with hour 6, given the hormonal milieu?
19. In a migration assay, amoeboid cells were subjected to alternating periods of high and low mechanical confinement. Under high confinement, cells displayed short-lived, actin-rich protrusions and maintained movement by repeatedly forming new protrusions. Under low confinement, protrusions persisted longer and cells migrated with fewer protrusion cycles. When actin polymerization was partially inhibited, the largest drop in net displacement occurred specifically under high confinement. Based on the vignette, which conclusion about actin dynamics is most consistent?
20. A population of insects experiences a sudden shift from green vegetation to brown vegetation after an invasive plant spreads. Body color is heritable and varies continuously. In field mark–recapture studies, darker individuals have higher survival on brown vegetation due to reduced predation, and the mean body color becomes darker over successive generations. Which statement best reflects the process of natural selection described?

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