Practice Test 2 - 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 comparative physiology study, Species A is a desert rodent and Species B is a freshwater rodent. Both are placed on the same low-water diet for 72 hours with identical solute intake. Species A maintains plasma osmolality near baseline and produces very concentrated urine, whereas Species B develops rising plasma osmolality and cannot concentrate urine to the same degree. Which mechanism is most consistent with maintaining homeostasis under these conditions in Species A compared with Species B?
2. A comparative experiment evaluates two subviral particles. In preparation A, infectivity is retained after RNase treatment but reduced after protease treatment; infected neurons accumulate protease-resistant aggregates of a host protein. In preparation B, infectivity is lost after RNase treatment but retained after protease treatment; infected plant cells show abundant 21–24 nt small RNAs mapping to the agent sequence. Which statement most accurately describes the impact of prions on cellular function as supported by preparation A?
3. Researchers examined how changing ion channel permeability alters membrane potential in a non-spiking epithelial cell. The cell's resting membrane potential was $-60\ \text{mV}$. A drug was applied that selectively increased K$^+$ channel open probability without directly affecting Na$^+$ or Cl$^-$ channels. Ion concentrations (mM) were: K$^+$ in 140, out 5; Na$^+$ in 15, out 145; Cl$^-$ in 20, out 110. Immediately after drug addition, $V_m$ shifted to $-80\ \text{mV}$. Which statement best explains the changes in membrane potential observed?
4. Clinicians treated a subset of patients with a CDK4/6 inhibitor. Tumor biopsies taken 2 weeks after therapy showed reduced phosphorylation of Rb and decreased expression of E2F target genes in responders, but no change in these markers in non-responders. Sequencing revealed that many non-responders carried deletions of the RB1 gene. Which interpretation is most consistent with the mechanism of the drug and the biomarker data?
5. A purified 90-aa cytosolic enzyme is folded at pH 7.4 in 150 mM NaCl. A single-site variant replaces a solvent-exposed residue with a side chain that increases hydrophobic surface area without adding ionizable groups. Which substitution is most consistent with increasing the tendency of that position to become buried in the protein interior? General amino acid: $\mathrm{H_3N^+-CH(R)-COO^-}$
6. Researchers administered exogenous cortisol to healthy subjects for 5 days and then measured endogenous ACTH and cortisol responses to a standardized psychological stressor. Compared with placebo, the cortisol-treated group showed blunted ACTH release during the stressor and a smaller rise in endogenous cortisol. Which outcome would be expected if the neural stress input to the hypothalamus is intact but endocrine feedback is altered by exogenous hormone?
7. In an experiment on co-translational quality control, a eukaryotic cell line was treated with a compound that prevents release factor–mediated hydrolysis of the peptidyl-tRNA linkage at stop codons but does not affect elongation. Western blot showed reduced free full-length protein and increased high-molecular-weight material that shifted after RNase treatment. Which statement best distinguishes peptide bonds in the primary chain from the linkage being blocked? (Assume RNase removes tRNA.)
8. A newly identified Virus Y was examined for genome type using nuclease protection assays. Intact virions were resistant to both DNase I and RNase A. After capsid disruption, the extracted genome was degraded by RNase A but not by DNase I. When infected cells were treated with a drug that inhibits RNA-dependent RNA polymerase (RdRp), production of new virions decreased by 95%. Based on these results, which classification best fits Virus Y?
9. A metabolic experiment tests inhibition of pyruvate dehydrogenase (PDH) in an aerobic bacterium at 30°C, pH 7.0, with glucose as the carbon source. After 45 minutes, intracellular pyruvate and acetyl-CoA are measured. Table (relative units): Condition: Control, +PDH inhibitor Pyruvate: 100, 260 Acetyl-CoA: 100, 35 Which metabolic change is most consistent with the data?
10. A conceptual model of glucose uptake in the small intestine includes two membrane transport steps for an enterocyte: (1) apical uptake from lumen by a Na$^+$/glucose cotransporter (SGLT), which uses the Na$^+$ gradient; (2) basolateral exit to blood by a facilitative glucose transporter (GLUT). The Na$^+$ gradient is maintained by the basolateral Na$^+$/K$^+$-ATPase. Assume luminal glucose is present. Which result would be expected if the Na$^+$/K$^+$-ATPase is inhibited in enterocytes?
11. A researcher records from a neuron and observes that opening a particular ligand-gated channel produces a membrane potential change that reverses direction near 0 mV (inward current at −60 mV, outward current at +20 mV). The channel is permeable to Na$^+$ and K$^+$ but not Cl$^−$. Which statement best explains the changes in membrane potential observed?
12. Chondrocytes were cultured in 3D matrices containing either high-density hyaluronan or low-density hyaluronan, with identical collagen content. High-density hyaluronan increased cell rounding and decreased proliferation; blocking CD44 (a hyaluronan receptor) restored proliferation without changing cell–cell contact frequency. Which conclusion is most consistent with the vignette?
13. A genetics group studies two autosomal loci in Drosophila melanogaster: gene b (body color) and gene c (wing shape). A female heterozygote in coupling phase (BC/bc) is testcrossed to a male bc/bc. Offspring phenotypes are counted: BC: 410 bc: 395 Bc: 95 bC: 100 No other loci are considered. Which explanation best accounts for the inheritance pattern observed?
14. A patient with chronic diarrhea presents with low blood pressure, tachycardia, and decreased urine output. Labs show low plasma bicarbonate and low blood pH. Which change would most likely restore homeostasis in the described system?
15. A mouse line is heterozygous for a paracentric inversion on chromosome 7 (one homolog inverted relative to the other). In meiosis I, synapsis occurs via formation of an inversion loop. The lab observes that some meiotic products are not recovered as viable offspring when the heterozygote is crossed to a wild-type mouse. Which outcome is most consistent with crossing over occurring within the inversion loop during meiosis I?
16. A psychiatry trial measured salivary cortisol after administering an SSRI for 8 weeks in patients with elevated baseline anxiety. Compared with placebo, the SSRI group showed reduced waking cortisol and reduced cortisol reactivity to a standardized social stress task. Which mechanism is most consistent with neuroendocrine integration that could explain the reduced cortisol response over time?
17. In a comparative study, researchers noted that spermatogenesis produced many small motile cells, while oogenesis produced one large cell per precursor. Both processes involved meiosis. Based on the information, which statement is most consistent with principles of gamete formation rather than mitosis?
18. Researchers label two different membrane proteins: Protein 1 is a single-pass transmembrane receptor; Protein 2 is a cytosolic enzyme that associates with the inner leaflet via a lipid anchor. Both show lateral movement in the membrane plane, but Protein 2 can be released by cleaving the lipid anchor. Which statement best reflects the fluid mosaic model?
19. In a clamp study, researchers infuse insulin at a constant rate while adjusting glucose infusion to maintain plasma glucose at 90 mg/dL. When insulin infusion is doubled, the required glucose infusion rate increases to maintain the same glucose level. Which of the following best describes the homeostatic principle demonstrated by this setup?
20. In a study of chromosome alignment, researchers cultured cells in low-dose nocodazole that partially suppresses microtubule polymerization but does not eliminate spindles. Many cells formed bipolar spindles with several chromosomes failing to align at the metaphase plate. These cells showed persistent SAC signaling and delayed anaphase onset. When nocodazole was washed out, chromosomes rapidly aligned and anaphase proceeded. Which event would be expected during metaphase in the control (untreated) cells but is least likely in the nocodazole-treated cells?

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