Try five real MCAT questions.

A diffusion coefficient D appears in the flux relation $J=D\cdot (\Delta C/\Delta x)$, where J has units $\mathrm{mol}\cdot {\mathrm{m}}^{-2}\cdot {\mathrm{s}}^{-1}$, $\Delta C$ has units $\mathrm{mol}\cdot {\mathrm{m}}^{-3}$, and $\Delta x$ has units $\mathrm{m}$. What are the SI base-unit dimensions of D?

• A$\mathrm{m}\cdot {\mathrm{s}}^{-1}$
• B${\mathrm{m}}^2\cdot {\mathrm{s}}^{-1}$
• C$\mathrm{mol}\cdot {\mathrm{m}}^{-1}\cdot {\mathrm{s}}^{-1}$
• D$\mathrm{mol}\cdot {\mathrm{m}}^{-2}\cdot {\mathrm{s}}^{-1}$

A researcher claims the data show that CDK4/6 activity is required for cells to reach the $G_2/M$ (4n) state after release from serum starvation. Which comparison of values from the figure most directly supports this claim?

• AAt 24 h, the 10% serum treatment reaches 31.0% 4n cells, whereas the 10% serum + CDK4/6 inhibitor treatment reaches only 6.0%, near the 5.0% control value.
• BAt 0 h, all three treatments show 4.0% 4n cells, confirming that the populations were identically synchronized before release.
• CAt 12 h, the 10% serum treatment shows 9.0% 4n cells while the control shows 4.0%, a difference that grows over time.
• DAcross all time points, the 0.1% serum control rises only from 4.0% to 5.0%, showing that mitogen withdrawal prevents 4n accumulation.

A researcher measures equilibrium binding of a small ligand to a transport protein. The fraction bound, Y, at several free-ligand concentrations [L]: [L] = 0.5 µM, Y = 0.20; [L] = 1.0 µM, Y = 0.33; [L] = 2.0 µM, Y = 0.50; [L] = 4.0 µM, Y = 0.67; [L] = 8.0 µM, Y = 0.80. A Hill plot yields a slope of 1.0. Which conclusion is best supported?

• ABinds cooperatively, each ligand increasing affinity for the next
• B$K_d\approx 4.0$ µM
• CNoncooperative binding with $K_d\approx 2.0$ µM
• DSigmoidal binding curve indicating multiple interacting sites

A physician models blood flow through an artery using Poiseuille's law, where flow rate Q depends on the pressure difference $\Delta P$, radius r, length L, and viscosity $\eta$. During inflammation a segment narrows so its radius halves, with $\Delta P$ and all else unchanged. By what factor does Q change?

• Adecreases to $\frac{1}{2}$
• Bdecreases to $\frac{1}{4}$
• Cdecreases to $\frac{1}{16}$
• Ddecreases to $\frac{1}{32}$

Outer hair cells provide cochlear amplification strongest near a fiber's characteristic frequency (CF) and weakest far from it. Using the tuning-curve data shown, which comparison of the control-to-furosemide threshold change best supports the claim that furosemide selectively abolished this amplification?

• AThreshold rose 38 dB at 8 kHz but only 2 dB at 4 kHz — the largest increase at the CF
• BThreshold rose 38 dB at both 8 and 14 kHz (uniform loss)
• CThreshold rose 2 dB at 4 kHz but fell 11 dB at 11 kHz
• DThreshold rose 30 dB at 7 kHz but 38 dB at 14 kHz (loss greatest far from CF)