Practice Test 7 - MCAT Chemical and Physical Foundations of Biological Systems

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

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1. A hospital Doppler device displays the reflected signal from blood as a higher frequency than the transmitted ultrasound. The technologist suspects the device is malfunctioning because "reflections can't change frequency." Which statement is most consistent with the Doppler Effect in this biologically relevant context?
2. A study tests a new drug that may reduce migraine frequency. During recruitment, the investigator emphasizes potential benefits and says, "We don't expect major problems," but does not mention that a small fraction of participants in an earlier phase reported mood changes. The investigator reasons, "Those events weren't definitively linked to the drug." Based on the scenario, what is the most ethical course of action?
3. A lab evaluates keto–enol tautomerization of a 1,3-dicarbonyl used as a chelator. Ethyl acetoacetate (CH$_3$COCH$_2$COOEt) is dissolved in ethanol with a catalytic amount of base. UV-vis shows increased absorbance consistent with formation of a conjugated enol. The central concept is enolization enhanced by adjacent carbonyl stabilization. Which site is most likely deprotonated to initiate enol formation?
4. A soluble enzyme is stabilized by a buried salt bridge between a Lys side chain and an Asp side chain in its tertiary structure. A patient-derived variant substitutes Asp with Asn at this position. Differential scanning calorimetry shows a lower melting temperature ($T_m$) but near-normal secondary structure at room temperature. What is the most likely consequence of the mutation described on protein stability?
5. A researcher charges a capacitor to $V_0$ using an ideal battery, then keeps the capacitor connected to the battery while a dielectric slab is inserted fully between the plates, increasing capacitance to $\kappa C_0$ (with $\kappa>1$). Which statement best reflects the principle of capacitance in this system? (Use $Q=CV$.)
6. An organic synthesis step in a radiotracer preparation uses nucleophilic addition of hydride to a carbonyl. A solution of cyclohexanone is treated with NaBH$_4$ in methanol at 0°C, then quenched with water. The chemist expects reduction of the carbonyl without changing the carbon skeleton. Which molecule is most likely formed from the reaction described?
7. A materials science lab tested the hypothesis that increasing ionic strength increases the conductivity of an aqueous electrolyte. Four beakers contained 100 mL of deionized water with NaCl added to final concentrations of 0 mM, 25 mM, 50 mM, and 100 mM. Conductivity was measured at 25°C using the same calibrated probe after gentle stirring for 60 s. Results showed a monotonic increase in conductivity with NaCl concentration. To evaluate whether the study supports the stated hypothesis, which conclusion is most appropriate given the design and results?
8. A spectroscopy lab studying a DNA-binding dye observes a strong absorption band attributed to a $\pi \to \pi^*$ electronic excitation localized on an aromatic ring system. The excitation is modeled as promoting an electron into a higher-energy molecular orbital without changing its spin. Which principle best explains the observed electron behavior?
9. A conductivity experiment compares 0.10 M solutions of NaCl, NaCH$_3$COO, and CH$_3$COOH at 25°C. Assume NaCl and NaCH$_3$COO fully dissociate; CH$_3$COOH partially dissociates: CH$_3$COOH $\rightleftharpoons$ H$^+$ + CH$_3$COO$^-$. Which statement best reflects the behavior of ions in the solution?
10. In a photoelectric experiment, monochromatic light is shone on a clean sodium surface in vacuum while the intensity is held constant. The stopping potential $V_s$ is measured for each frequency. Constants: $h = 6.63\times10^{-34}\ \text{J}\cdot\text{s}$, $e = 1.60\times10^{-19}\ \text{C}$. Which outcome would be expected if the light frequency is increased from just below to just above the threshold frequency $f_0$ for sodium?
11. A racemic tertiary amine is separated on a chiral HPLC column. The analyst switches from UV detection to mass spectrometry detection (only variable changed). Retention times and peak resolution are unchanged, but peak areas differ from the UV run. Based on the technique used, which conclusion is most consistent with the results?
12. In a microfluidic electrophoresis assay, a DNA fragment with net charge $q=-3.2\times10^{-19}\ \text{C}$ enters a region of uniform electric field $\vec E = 2.0\times10^{4}\ \text{N/C}$ directed to the right. Neglect fluid drag and gravity. Based on the setup, which outcome is most likely for the DNA fragment's initial acceleration?
13. A PET tracer sample contains $^{18}\text{F}$ with half-life 110 min. The sample is transported for 220 min before use. Based on the decay model, what outcome is most likely for the remaining activity (ignoring biological clearance)?
14. A lab technician pushes a cart up a short ramp, starting and ending at rest. Compared with a frictionless ramp, the technician reports needing to apply more force over the same displacement when the ramp surface is rough. Principle tested: nonconservative work and mechanical energy. Which statement is most consistent with the added work input?
15. A sealed reaction chamber contains an enzyme system that converts one dissolved substrate molecule into two dissolved product molecules at constant temperature. The chamber is closed (no matter exchange) and rigid (constant volume). Considering the second law of thermodynamics and entropy, which conclusion about the entropy change of the system is most consistent with this scenario?
16. In a study of kidney stone prevention, researchers modeled calcium phosphate precipitation by preparing a buffered aqueous solution at 37°C containing $1.0\ \text{mM}$ $\text{Ca}^{2+}$ and $0.60\ \text{mM}$ $\text{PO}_4^{3-}$ (assume $\text{PO}_4^{3-}$ is the relevant phosphate species at this pH). For $\text{Ca}_3(\text{PO}_4)_2(s) \rightleftharpoons 3\text{Ca}^{2+} + 2\text{PO}_4^{3-}$, $K_{sp}=2.0\times 10^{-29}$ at 37°C. The team then adds $\text{CaCl}_2$ to raise free $\text{Ca}^{2+}$ without changing volume appreciably. Which shift in equilibrium is most consistent with adding the common ion $\text{Ca}^{2+}$ under these conditions?
17. A stationary observer stands between two identical sound sources emitting the same frequency. Source X approaches the observer while Source Y recedes from the observer along the same line. Which outcome is most consistent with the Doppler Effect?
18. To probe carbonyl nucleophilic addition in a drug–protein adduct model, researchers incubate an aldehyde-containing ligand (R–CHO) with a lysine side-chain mimic (n-butylamine, 20 mM) at pH 7.4. After 1 hour, IR spectroscopy shows decreased C=O stretch intensity and appearance of a new C=N stretch. No external reducing agent is present. Which product is most likely formed from the reaction described?
19. A $10\,\mu\text{F}$ capacitor is connected directly across a $9\,\text{V}$ battery and allowed to reach steady state. Without disconnecting the battery, a dielectric is inserted that doubles the capacitance. Which condition would lead to the highest voltage across the capacitor immediately after insertion? (Assume ideal battery; $V$ across elements in parallel equals battery voltage.)
20. A PI runs a study on a new wearable sensor that estimates blood oxygenation using optical absorption. A startup company that sells the sensor pays for the study and offers to review the manuscript before submission, with the right to delay publication for "business reasons." The PI thinks, "We need the funding, and delays are normal." Which factor is most critical to consider in evaluating the ethical implications?

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