Practice Test 1 - 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. A lab is attempting to classify an unknown virus from a skin lesion. Infectivity drops >99.9% after brief exposure to ether, and viral entry into cells is inhibited by a peptide that blocks membrane fusion at neutral pH. In contrast, a drug that inhibits clathrin-mediated endocytosis has minimal effect on entry. Based on these data, what is most likely the method of classification for this virus?
2. A population of marine algae has two alleles at a locus affecting tolerance to high light: L1 and L2. A shift to clearer water increases light intensity. Algae with L2 have higher growth rate in high light, and allele L2 increases from 0.10 to 0.48 over 8 generations. What adaptive advantage is most likely to occur?
3. Researchers administer a single dose of an ACE inhibitor to volunteers with mild hypertension. Two hours later, plasma angiotensin II is decreased, and blood pressure is reduced. Based on the described intervention, which interaction is critical for maintaining homeostasis of blood pressure and extracellular fluid volume?
4. Fibroblasts were treated with a drug that prevents formation of COPII-coated vesicles. COPII vesicles normally bud from the endoplasmic reticulum (ER), a membrane-bound organelle where many secreted and membrane proteins enter the secretory pathway. After treatment, a newly synthesized plasma membrane receptor accumulated in the ER and was not detected at the cell surface. Which outcome best exemplifies the role of ER-to-Golgi trafficking in this scenario?
5. A strain lacking functional RecA is exposed to purified DNA carrying a wild-type allele that restores an auxotrophy marker. After treatment, the strain shows transient expression of the marker for several generations, but the phenotype is lost upon further growth without selection. No plasmid sequences are present in the donor DNA. Which process best explains the observed genetic change?
6. Virus CC is an RNA virus. In infected cells, viral mRNA appears only after a delay, and purified virions contain an enzyme that synthesizes RNA from an RNA template. When purified genomic RNA is transfected into cells, no viral proteins are detected. Which classification best fits the virus described?
7. A lab engineered a cytosolic enzyme (Enzyme Y) with a single Tyr phosphorylation site required for catalytic activity. After stimulating cells with a growth factor, Enzyme Y activity increased within minutes without a change in Enzyme Y protein abundance. Treatment with a broad tyrosine kinase inhibitor blocked the activity increase. How does phosphorylation most likely alter Enzyme Y function in this context?
8. A research group perturbs cell migration during early gastrulation in an amphibian embryo by transiently increasing extracellular matrix cross-linking. Imaging shows that many cells fail to move inward; instead, they remain in the outer sheet that normally gives rise to the epidermis and neural tissue. The embryo later exhibits markedly reduced formation of a coelom and a severe deficit of blood islands. Which outcome would be expected if inward migration through the blastopore is specifically reduced during gastrulation?
9. Researchers compare early cellular changes after exposure to either a prion preparation (protein-only infectious material) or a viroid preparation (small circular RNA). In neurons exposed to the protein-only agent, a host membrane glycoprotein shifts to an insoluble fraction and forms punctate intracellular aggregates. In plant cells exposed to the circular RNA, no protein aggregates are observed, but specific host mRNAs decrease alongside increased siRNAs. Which statement best links each subviral particle to its primary implication for cellular organization?
10. In a hypoglycemia model (150–250 words), isolated liver cells were supplied with alanine as the primary carbon source under high glucagon. Investigators observed increased conversion of alanine to pyruvate and then increased glucose output. When pyruvate carboxylase was inhibited, glucose output fell and pyruvate accumulated despite adequate ATP. Reaction context: Pyruvate + CO$_2$ + ATP $\xrightarrow{\textit{pyruvate carboxylase}}$ Oxaloacetate Which prediction is most aligned with inhibition of pyruvate carboxylase in this setting?
11. Researchers recorded miniature excitatory postsynaptic currents (mEPSCs) from a postsynaptic neuron in the presence of tetrodotoxin to eliminate presynaptic action potentials. After applying a drug that reduces presynaptic voltage-gated Ca$^{2+}$ channel opening, they observed fewer mEPSC events per unit time, but the average mEPSC amplitude was unchanged. Concept tested: neurotransmitter release probability vs postsynaptic receptor responsiveness. Which interpretation best explains these findings?
12. In an endocrine ward, a patient with central diabetes insipidus is treated with desmopressin (a V2 receptor agonist). The patient's thirst decreases and urine osmolality increases. The clinician notes that the therapy bypasses a specific neuroendocrine step. Which step is most directly bypassed by desmopressin in this disorder?
13. A population of insects experiences a sudden cold snap each winter. A cuticle-lipid trait affects cold tolerance: genotype CC yields high tolerance, Cc intermediate, and cc low. After multiple winters, researchers observe that the frequency of C increases and that CC individuals contribute disproportionately to the next generation. Which statement best reflects the process of natural selection described?
14. A patient has exercise intolerance and recurrent muscle cramps. A biopsy shows markedly increased glycogen in skeletal muscle with low lactate production during an ischemic forearm test. The concept being tested is tissue-specific consequences of impaired glycogen mobilization. Based on this scenario, which outcome is most consistent with a deficiency in muscle glycogen phosphorylase? Assume liver enzymes are normal.
15. A student analyzes offspring from a cross involving two autosomal traits with complete dominance: A/a and B/b. The student observes four phenotype classes, but one class is much more common than the others. If the genes are assumed to assort independently under Mendelian inheritance, which conclusion is most consistent?
16. To probe anaphase mechanics, researchers selectively inhibited microtubule depolymerization at kinetochores while leaving depolymerization at spindle poles largely intact. During anaphase, chromatids showed reduced movement toward poles early in anaphase, but pole-to-pole spindle elongation proceeded similarly to controls. Which conclusion about the forces driving chromosome movement is most consistent with the data?
17. Researchers differentiated human induced pluripotent stem cells (iPSCs) into two lineages using brief exposure to different morphogens. After 48 hours, RNA-seq showed that Lineage A strongly expressed transcription factor TF-A and low levels of TF-B, while Lineage B showed the opposite pattern. Whole-genome sequencing confirmed both lineages were genetically identical to the starting iPSCs. The cellular principle assessed is that differential gene expression, driven by regulatory networks, can produce distinct cell types without changes to DNA sequence. Which mechanism best explains the stable divergence in lineage identity observed after the morphogen pulse?
18. A laboratory study assessed gas exchange efficiency in isolated perfused rat lungs under two ventilatory patterns while holding inspired O2 constant. Pattern 1 used shallow, rapid breaths; Pattern 2 used slower, deeper breaths. Total minute ventilation was matched between patterns. Investigators measured end-capillary $P_{O_2}$ and noted lower values with Pattern 1 despite identical minute ventilation. Which explanation best accounts for the reduced oxygenation with shallow, rapid breathing?
19. A patient has a mutation that reduces activity of medium-chain acyl-CoA dehydrogenase (MCAD). During an overnight fast, plasma acylcarnitine profiling shows elevated medium-chain acylcarnitines, while urinary ketones are low despite hypoglycemia. Based on the experimental setup, what is the most likely effect of the mutation on metabolism?
20. A polymerase extends a DNA primer on a DNA template during replication. The next template base to be read is T. The reaction is performed under conditions where only one deoxynucleoside triphosphate is available. Based on base pairing principles, which available nucleotide would allow correct incorporation and continued extension?

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