Practice Test 9 - MCAT Biological and Biochemical Foundations of Living Systems

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

After a primary exposure to a novel protein antigen, serum antibody titers rise slowly and peak around day 14. After a booster with the same antigen months later, titers rise rapidly and peak higher. Which outcome would be expected following antigen exposure that best explains this pattern?
A metabolic pathway experiment tests an inhibitor of dihydrofolate reductase (DHFR) in a bacterium growing at 37°C, pH 7.2, in minimal medium. After 90 minutes, intracellular dTMP is measured. Table (intracellular dTMP, relative units): Control: 100 +DHFR inhibitor: 15 Which metabolic change is most consistent with the data?
A study examined how changes in pulmonary capillary transit time affect arterial oxygenation during moderate exercise. In one trial, subjects exercised normally; in another, heart rate was pharmacologically increased, shortening capillary transit time while ventilation and inspired O $_2$ fraction were unchanged. Arterial $P_{O_2}$ fell slightly in the shortened-transit trial. Which explanation is most consistent with the observed change? Physiological concept: diffusion equilibration time for O $_2$ versus perfusion effects in the pulmonary circulation.
A biomechanics lab evaluated articular cartilage, a connective tissue in which chondrocytes (matrix-producing eukaryotic cells) are embedded in an extracellular matrix rich in collagen and proteoglycans (negatively charged macromolecules that attract water). Cartilage samples were treated with an enzyme that selectively degrades proteoglycans while leaving collagen largely intact. After treatment, samples showed increased compressibility under load and reduced ability to recover thickness after unloading, despite unchanged chondrocyte viability. Which statement best explains the role of this tissue's organization in resisting compression?
An experimental drug blocks apical Cl− secretion in intestinal crypt cells. Definitions: Cl− secretion into the lumen promotes Na+ and water movement into the lumen; net secretion increases luminal water content. Which result would be expected if apical Cl− secretion is inhibited while absorptive processes are unchanged?
Investigators stimulated the vagus nerve electrically in anesthetized animals while monitoring plasma gastrin and gastric acid output. Vagal stimulation increased acid output within minutes and increased plasma gastrin modestly. When animals were pretreated with a muscarinic receptor antagonist, the acid output response was largely abolished, but the gastrin increase persisted. Which outcome would be expected if the primary neural signal were altered in a way that selectively blocks cholinergic synapses but not peptide-mediated endocrine release?
A plant pathology lab investigates subviral particles in greenhouse tomatoes with stunted growth and leaf curling. Total nucleic acid extracts from symptomatic plants transmit disease to healthy plants after mechanical abrasion, even when the extract is treated with protease. However, RNase treatment abolishes transmission. Sequencing of the infectious fraction reveals a small, circular, noncoding RNA with extensive intramolecular base pairing and no detectable protein-coding capacity. In infected plants, small interfering RNAs (siRNAs) complementary to the circular RNA increase, and several host mRNAs involved in chloroplast development decrease. Based on the vignette, what is most consistent with viroid replication and its cellular consequence?
A field study of a diploid amphibian quantified genotype frequencies at two loci in offspring from multiple mating pairs. The loci were on different chromosomes. Across families, offspring frequently carried allele combinations not present together on either parental homolog, yet within each family the four possible allele combinations appeared in roughly equal proportions when averaged over many offspring. No evidence supported postzygotic selection against any genotype. Based on the study, what is the most significant source of genetic variation in meiosis explaining the near-equal representation of all allele combinations?
A virology group examined viral entry mediated by a viral surface protein that binds a host cell receptor. A host receptor mutant lacking its extracellular binding site showed normal expression at the plasma membrane but markedly reduced viral attachment and entry. No intracellular signaling defects were detected during the short attachment assay. Based on the scenario, which outcome is most consistent with the receptor's function in this context?
Virus U and Virus V are compared for environmental persistence. After drying on stainless steel for 24 h, Virus U retains 40% infectivity, while Virus V retains <1%. Detergent treatment reduces Virus V infectivity by 4 logs but reduces Virus U by <0.2 logs. Based on the passage, how does the structure influence pathogenicity?
Two tumor cell lines are treated with the same CDK4/6 inhibitor. Line 1 expresses functional Rb; Line 2 has an RB1 loss-of-function mutation. After treatment, Line 1 shows decreased S-phase fraction and slower growth, while Line 2 shows little change in S-phase fraction or growth rate. Which conclusion is most consistent with these results?
A comparative study examined how the nucleus and the rough ER coordinate expression of a membrane receptor. The receptor's mRNA was abundant, and ribosome profiling indicated active translation. However, a mutation introduced a long hydrophobic segment that caused the nascent receptor to stall during insertion into the ER membrane, triggering ER-associated degradation (ERAD). ERAD is an ER quality-control process that retrotranslocates misfolded or stalled proteins to the cytosol for ubiquitin-dependent proteasomal degradation. Despite high mRNA, receptor levels at the plasma membrane were low. What cellular change is most consistent with increased ERAD activity in this scenario?
Researchers study how the microenvironment influences human pluripotent stem cell behavior. Cells are grown either on a synthetic surface presenting an adhesion peptide (Condition 1) or embedded in a soft hydrogel that limits spreading (Condition 2). After 5 days, the team measures colony morphology and expression of a pluripotency marker (NANOG). They also test self-renewal by dissociating colonies and reseeding single cells in the same condition. Which result would best indicate that the microenvironment in Condition 2 is reducing pluripotency rather than merely slowing proliferation?
A diagnostic lab uses PCR to detect a viral DNA sequence in patient swabs. To reduce false positives from carryover contamination, the lab switches from dTTP to dUTP in PCR reactions and treats reaction mixes with uracil-DNA glycosylase (UDG) before amplification. The goal is to degrade contaminating PCR products from prior runs while preserving authentic viral DNA in new specimens. Which outcome is most consistent with this PCR contamination-control strategy?
In a signaling study, Protein R is phosphorylated at a single threonine residue following receptor activation. The phosphorylated form shows increased binding to a downstream effector in co-immunoprecipitation assays. A phospho-dead Thr→Ala mutant fails to bind the effector. How does phosphorylation most likely alter Protein R function?
A frog population occupies two ponds connected by a seasonal stream. At an autosomal locus with alleles J and j, each pond individually shows genotype frequencies consistent with Hardy-Weinberg expectations when sampled in isolation. When researchers pool samples from both ponds into a single dataset, they observe fewer heterozygotes than expected from the pooled allele frequencies. Hardy-Weinberg equilibrium assumes: (i) a single randomly mating population, (ii) no selection, (iii) no mutation, (iv) no migration, and (v) large population size. Which factor most likely explains the deviation in the pooled dataset?
In a set of tumor-derived cells, sequencing shows a gain-of-function mutation in cyclin D that prevents its ubiquitination and degradation. Compared with cells expressing degradable cyclin D, mutant cells show persistently high cyclin D levels and increased Rb phosphorylation throughout the cell cycle. Which outcome is most consistent with this mutation? Core concept: sustained cyclin D can keep CDK4/6 active, maintaining Rb phosphorylation and promoting continued G1/S progression.
In a human embryology lab, a gastrulating embryo is modeled as three concentric layers: an outer epithelial sheet, a middle migratory layer, and an inner layer that will line the primitive gut. A disruption is induced that selectively prevents internalization of cells through the primitive streak, while cells on the surface continue to proliferate and spread. Which developmental outcome is most consistent with this disruption?
A clinical study evaluates a patient with reduced release of cholecystokinin (CCK) from the small intestine after a fatty meal. Provided definitions: CCK increases pancreatic enzyme secretion and stimulates gallbladder contraction; bile salts emulsify dietary lipids to increase micelle formation; pancreatic lipase acts at the lipid–water interface. The patient consumes a meal containing 60 g triglyceride. Which result would be expected if CCK release is deficient?
Malate dehydrogenase is inhibited. Immediately, which pair of TCA changes occurs?

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