I85PREP
⌘ K
MCAT study materials
MCAT study resources

MCAT practice questions

One question at a time. Build your MCAT understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Natural sciences concepts

A solution has a hydrogen ion concentration of 1 × 10⁻⁴ M. What is the pH of this solution?

  1. 10
  2. −4
  3. 14
  4. 4 Correct answer.

Explanation

pH is defined as the negative base-10 logarithm of the hydrogen ion concentration, so pH = −log(10⁻⁴) = 4. A value of 10 would correspond to a basic solution with [H⁺] = 10⁻¹⁰ M.

Question 2Natural sciences concepts

Which intermolecular force is primarily responsible for the unusually high boiling point of water compared with hydrogen sulfide?

  1. Dipole–dipole interactions
  2. Hydrogen bonding Correct answer.
  3. Ion–dipole interactions
  4. London dispersion forces

Explanation

Water's O–H bonds allow each molecule to donate and accept hydrogen bonds through the electronegative oxygen and its lone pairs, producing strong intermolecular attraction. Hydrogen sulfide has weaker hydrogen bonding because sulfur is larger and less electronegative.

Question 3Natural sciences concepts

In a population at Hardy–Weinberg equilibrium, the frequency of the recessive allele is 0.3. What fraction of the population is expected to be heterozygous?

  1. 0.09
  2. 0.21
  3. 0.42 Correct answer.
  4. 0.49

Explanation

With q = 0.3, p = 0.7, and heterozygote frequency is 2pq = 2(0.7)(0.3) = 0.42. The value 0.09 is q², the homozygous recessive frequency.

Question 4Natural sciences concepts

A reaction has a negative ΔH and a positive ΔS. Which statement about its spontaneity is correct?

  1. It is spontaneous at all temperatures Correct answer.
  2. It is spontaneous only at high temperature
  3. It is spontaneous only at low temperature
  4. It is nonspontaneous at all temperatures

Explanation

The Gibbs free energy change is ΔG = ΔH − TΔS. A negative ΔH and positive ΔS make both terms negative at any positive temperature, so ΔG is always negative and the reaction is spontaneous throughout.

Question 5Natural sciences concepts

Which type of bond is formed when a cysteine side chain interacts with another cysteine side chain to stabilize a protein's tertiary structure?

  1. Phosphodiester bond
  2. Disulfide bond Correct answer.
  3. Peptide bond
  4. Hydrogen bond

Explanation

Two cysteine residues can be oxidized to form a covalent disulfide bond between their sulfhydryl groups, which stabilizes tertiary and quaternary structure. Peptide bonds join amino acids in the backbone, and phosphodiester bonds belong to nucleic acids.

Question 6Natural sciences concepts

A block of mass 2 kg is pushed across a frictionless surface by a constant horizontal force of 10 N. What is the acceleration of the block?

  1. 20 m/s²
  2. 5 m/s² Correct answer.
  3. 0.2 m/s²
  4. 10 m/s²

Explanation

Newton's second law gives a = F/m = 10 N / 2 kg = 5 m/s². Multiplying mass by force or inverting the ratio produces the other numeric choices.

Question 7Natural sciences concepts

During glycolysis, which enzyme catalyzes the committed step that phosphorylates fructose-6-phosphate?

  1. Pyruvate kinase
  2. Aldolase
  3. Phosphofructokinase-1 Correct answer.
  4. Hexokinase

Explanation

Phosphofructokinase-1 converts fructose-6-phosphate to fructose-1,6-bisphosphate and is the rate-limiting committed step of glycolysis. Hexokinase acts earlier on glucose, while aldolase and pyruvate kinase act downstream.

Question 8Natural sciences concepts

A light ray passes from air (n = 1.0) into glass (n = 1.5) at an angle of incidence of 30°. Which statement about the refracted ray is correct?

  1. It bends toward the normal and travels more slowly Correct answer.
  2. It bends away from the normal and travels faster
  3. It bends toward the normal and travels faster
  4. It continues straight without changing direction

Explanation

Snell's law, n₁sinθ₁ = n₂sinθ₂, shows that entering a higher-index medium decreases the refracted angle, so the ray bends toward the normal. The speed of light in the medium is v = c/n, so it slows in glass.

Question 9Natural sciences concepts

Which organelle is the primary site of ATP synthesis by oxidative phosphorylation in a eukaryotic cell?

  1. Mitochondrion Correct answer.
  2. Lysosome
  3. Rough endoplasmic reticulum
  4. Golgi apparatus

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which carry out oxidative phosphorylation. The other listed organelles serve protein processing, modification, and degradation.

Question 10Natural sciences concepts

A gas occupies 3 L at 300 K and constant pressure. What volume will it occupy at 600 K?

  1. 3 L
  2. 1.5 L
  3. 6 L Correct answer.
  4. 9 L

Explanation

Charles's law states that volume is directly proportional to absolute temperature at constant pressure, so V₂ = V₁(T₂/T₁) = 3 L × (600/300) = 6 L. Halving or tripling the volume misapplies the ratio.

Question 11Natural sciences concepts

In a cross between two individuals heterozygous for a single gene, what phenotypic ratio is expected among offspring if the alleles show complete dominance?

  1. 9:3:3:1
  2. 1:2:1
  3. 1:1
  4. 3:1 Correct answer.

Explanation

A monohybrid cross of Aa × Aa yields genotype frequencies of 1 AA : 2 Aa : 1 aa. With complete dominance, the AA and Aa classes share the dominant phenotype, giving a 3:1 phenotypic ratio.

Question 12Natural sciences concepts

A molecule has a tetrahedral electron geometry with four identical bonded atoms and no lone pairs on the central atom. What is its molecular geometry and polarity?

  1. Tetrahedral and polar
  2. Tetrahedral and nonpolar Correct answer.
  3. Trigonal pyramidal and polar
  4. Square planar and nonpolar

Explanation

Four identical bonds and no lone pairs give a symmetrical tetrahedral shape, and the bond dipoles cancel, making the molecule nonpolar. For example, methane fits this description.

Question 13Natural sciences concepts

Which statement best describes the effect of a competitive inhibitor on an enzyme-catalyzed reaction?

  1. It lowers both Km and Vmax
  2. It raises Km and leaves Vmax unchanged Correct answer.
  3. It raises both Km and Vmax
  4. It lowers Vmax and leaves Km unchanged

Explanation

A competitive inhibitor binds the active site, so higher substrate concentrations can outcompete it, leaving Vmax unchanged while increasing the apparent Km. Noncompetitive inhibition instead lowers Vmax.

Question 14Natural sciences concepts

A sound wave has a frequency of 500 Hz and a wavelength of 0.68 m in air. What is its approximate speed?

  1. 735 m/s
  2. 0.0014 m/s
  3. 170 m/s
  4. 340 m/s Correct answer.

Explanation

Wave speed equals frequency times wavelength, so v = 500 Hz × 0.68 m = 340 m/s. Dividing instead of multiplying or using the wrong operation yields the distractors.

Question 15Natural sciences concepts

In the nephron, in which segment is the majority of filtered glucose reabsorbed under normal conditions?

  1. Collecting duct
  2. Distal convoluted tubule
  3. Proximal convoluted tubule Correct answer.
  4. Descending limb of the loop of Henle

Explanation

Glucose is reabsorbed almost entirely in the proximal convoluted tubule via sodium-glucose cotransporters. The loop of Henle, distal tubule, and collecting duct handle water, ion, and hormone-regulated adjustments instead.

Question 16Natural sciences concepts

A reaction is first order with respect to reactant A. If the initial concentration of A is doubled while temperature is held constant, how does the initial rate change?

  1. It doubles Correct answer.
  2. It quadruples
  3. It halves
  4. It stays the same

Explanation

For a first-order reaction, rate = k[A]¹, so doubling [A] doubles the rate. Quadrupling would require second-order dependence on A.

Question 17Natural sciences concepts

Which part of the nervous system is primarily responsible for the fight-or-flight response?

  1. Parasympathetic division
  2. Enteric nervous system
  3. Sympathetic division Correct answer.
  4. Somatic motor division

Explanation

The sympathetic division of the autonomic nervous system mobilizes the body during stress by increasing heart rate, dilating airways, and redirecting blood flow. The parasympathetic division promotes rest-and-digest functions.

Question 18Natural sciences concepts

An object is placed 15 cm in front of a converging lens with a focal length of 10 cm. Where is the image formed?

  1. 6 cm on the same side as the object
  2. 15 cm on the same side as the object
  3. 10 cm on the opposite side of the lens
  4. 30 cm on the opposite side of the lens Correct answer.

Explanation

The thin lens equation 1/f = 1/do + 1/di gives 1/10 = 1/15 + 1/di, so 1/di = 1/30 and di = 30 cm. A positive image distance places a real image on the opposite side of the lens.

Question 19Natural sciences concepts

Which of the following best explains why the carbon–fluorine bond is more polar than the carbon–hydrogen bond?

  1. Carbon has a higher electronegativity than fluorine
  2. Fluorine is more electronegative than hydrogen Correct answer.
  3. Fluorine has a larger atomic radius than hydrogen
  4. The C–F bond is a double bond

Explanation

Bond polarity reflects the electronegativity difference between the bonded atoms. Fluorine's high electronegativity creates a larger dipole than hydrogen's much lower electronegativity.

Question 20Natural sciences concepts

During which phase of the cell cycle does DNA replication occur?

  1. M phase
  2. S phase Correct answer.
  3. G1 phase
  4. G2 phase

Explanation

DNA replication takes place during S phase, which lies between G1 and G2 in interphase. G1 and G2 are growth phases, and M phase is when chromosomes are segregated and the cell divides.

Question 21Natural sciences concepts

A buffer is prepared from equal concentrations of acetic acid and sodium acetate. If a small amount of strong acid is added, which component neutralizes it?

  1. Sodium ion
  2. Acetic acid
  3. Water
  4. Acetate ion Correct answer.

Explanation

In an acidic buffer, the conjugate base (acetate) neutralizes added strong acid by accepting protons to form more acetic acid. The weak acid component instead neutralizes added base.

Question 22Natural sciences concepts

Which of the following is a product of the light-dependent reactions of photosynthesis that is used in the Calvin cycle?

  1. Glucose and oxygen
  2. NADPH and ATP Correct answer.
  3. Carbon dioxide and water
  4. NADH and FADH₂

Explanation

The light reactions generate ATP and NADPH, which supply energy and reducing power for carbon fixation in the Calvin cycle. Oxygen is released as a byproduct, and glucose is a product of the Calvin cycle.

Question 23Natural sciences concepts

A 4 kg object moving at 3 m/s collides and sticks to a stationary 2 kg object. What is the speed of the combined mass immediately after the collision?

  1. 6 m/s
  2. 3 m/s
  3. 2 m/s Correct answer.
  4. 1 m/s

Explanation

Momentum is conserved in a perfectly inelastic collision: (4 kg)(3 m/s) = (6 kg)v, so v = 2 m/s. Kinetic energy is not conserved in this collision, but momentum is.

Question 24Natural sciences concepts

Which type of RNA carries amino acids to the ribosome during translation?

  1. Messenger RNA
  2. Transfer RNA Correct answer.
  3. Small nuclear RNA
  4. Ribosomal RNA

Explanation

Transfer RNA molecules pair their anticodons with mRNA codons and deliver the corresponding amino acids to the growing polypeptide chain. Messenger RNA carries the coding sequence, and ribosomal RNA forms part of the ribosome.

Question 25Natural sciences concepts

An unknown compound dissolves readily in water, conducts electricity in aqueous solution, and has a high melting point. Which class of compound best fits these observations?

  1. A noble gas
  2. An ionic compound Correct answer.
  3. A nonpolar molecular compound
  4. A molecular compound held by dispersion forces

Explanation

Ionic compounds dissociate into mobile ions that conduct electricity, and their strong electrostatic lattice gives high melting points and water solubility. Nonpolar molecular substances and noble gases do not conduct or dissolve well in water.

Question 26Natural sciences concepts

A reaction has ΔH = +45 kJ/mol and ΔS = +150 J/(mol·K). At approximately what temperature does the reaction become spontaneous under standard conditions?

  1. It is spontaneous at all temperatures
  2. 150 K
  3. 450 K
  4. 300 K Correct answer.

Explanation

Spontaneity requires ΔG = ΔH − TΔS < 0. Setting ΔH = TΔS gives T = 45,000 J/mol ÷ 150 J/(mol·K) = 300 K, so the reaction is spontaneous above 300 K.

Question 27Natural sciences concepts

Which intermolecular force is chiefly responsible for the unusually high boiling point of water relative to hydrogen sulfide?

  1. Dipole–dipole interactions
  2. Ion–dipole interactions
  3. Hydrogen bonding Correct answer.
  4. London dispersion forces

Explanation

Water's O–H bonds let each molecule donate and accept hydrogen bonds through its two lone pairs, giving an extensive network. This strong, directional interaction requires more energy to disrupt than the weaker forces available to H2S.

Question 28Natural sciences concepts

A solution contains 0.10 M weak acid HA with Ka = 1.0 × 10⁻⁵. What is the approximate pH of the solution?

  1. 7.0
  2. 5.0
  3. 3.0 Correct answer.
  4. 1.0

Explanation

For a weak acid, [H⁺] ≈ √(Ka·C) = √(1.0 × 10⁻⁵ × 0.10) = 1.0 × 10⁻³ M. Taking −log gives pH ≈ 3.0.

Question 29Natural sciences concepts

In an electrolytic cell, oxidation occurs at which electrode, and what is the sign of that electrode?

  1. Anode, which is positive Correct answer.
  2. Cathode, which is positive
  3. Anode, which is negative
  4. Cathode, which is negative

Explanation

Oxidation always occurs at the anode regardless of cell type. In an electrolytic cell an external power source makes the anode positive, drawing electrons away from the species being oxidized.

Question 30Natural sciences concepts

Which quantum number determines the orientation in space of an atomic orbital?

  1. Angular momentum quantum number l
  2. Spin quantum number ms
  3. Principal quantum number n
  4. Magnetic quantum number ml Correct answer.

Explanation

The magnetic quantum number ml distinguishes orbitals of a given subshell by their spatial orientation, ranging from −l to +l. The principal number sets energy level and the angular momentum number sets subshell shape.

Question 31Natural sciences concepts

Which of the following molecules is nonpolar despite containing polar bonds?

  1. H₂O
  2. NH₃
  3. HCl
  4. CO₂ Correct answer.

Explanation

CO₂ is linear, so its two C=O bond dipoles are equal and point in opposite directions, canceling to a net dipole of zero. Molecular geometry, not bond polarity alone, decides overall polarity.

Question 32Natural sciences concepts

Which of the following best describes the role of NADH in cellular respiration after it is produced in the citric acid cycle?

  1. It directly phosphorylates ADP to ATP in the cytosol
  2. It transports pyruvate into the mitochondrial matrix
  3. It donates electrons to the electron transport chain, driving proton pumping Correct answer.
  4. It accepts electrons from oxygen at complex IV

Explanation

NADH delivers high-energy electrons to complex I of the electron transport chain, and their passage through the chain pumps protons to build the gradient used for ATP synthesis. It is an electron carrier, not a direct phosphorylating agent.

Question 33Natural sciences concepts

In a population at Hardy–Weinberg equilibrium, the frequency of the recessive allele is 0.2. What proportion of the population is heterozygous?

  1. 0.64
  2. 0.32 Correct answer.
  3. 0.04
  4. 0.16

Explanation

With q = 0.2, p = 0.8. Heterozygote frequency is 2pq = 2(0.8)(0.2) = 0.32.

Question 34Natural sciences concepts

Which statement correctly describes the effect of a noncompetitive inhibitor on enzyme kinetics?

  1. It lowers Km and raises Vmax
  2. It raises both Vmax and Km
  3. It lowers Vmax without changing Km Correct answer.
  4. It raises Km without changing Vmax

Explanation

A noncompetitive inhibitor binds at a site other than the active site and reduces the fraction of functional enzyme, lowering Vmax. Because substrate binding affinity at the active site is largely unchanged, Km stays the same.

Question 35Natural sciences concepts

A woman who is a carrier of an X-linked recessive disorder has children with an unaffected man. What proportion of their sons is expected to be affected?

  1. 1/4
  2. 0
  3. All sons
  4. 1/2 Correct answer.

Explanation

Sons receive their single X chromosome from the mother, who has one mutant and one normal allele, so each son has a 1/2 chance of inheriting the mutant allele. The father contributes a Y chromosome, which carries no allele for this gene.

Question 36Natural sciences concepts

Which structural feature of the plasma membrane most directly limits the free diffusion of large polar molecules into a cell?

  1. The glycoprotein layer on the extracellular surface
  2. The hydrophobic fatty acid tails in the bilayer interior Correct answer.
  3. The hydrophilic phosphate heads of the phospholipids
  4. Peripheral membrane proteins on the cytoplasmic face

Explanation

The interior of the bilayer is hydrophobic, so large polar or charged molecules face an energetically unfavorable barrier there. Small nonpolar molecules cross readily, while large polar ones need transport proteins.

Question 37Natural sciences concepts

During which stage of cellular respiration is the majority of ATP produced?

  1. Oxidative phosphorylation Correct answer.
  2. Glycolysis
  3. Pyruvate oxidation
  4. The citric acid cycle

Explanation

Most ATP is generated by oxidative phosphorylation, where the proton gradient built by the electron transport chain drives ATP synthase. Glycolysis and the citric acid cycle yield comparatively small amounts through substrate-level phosphorylation.

Question 38Natural sciences concepts

A population of rabbits experiences a sudden drought that favors individuals with thicker fur. Over several generations, the average fur thickness increases. Which mechanism of evolution does this illustrate?

  1. Stabilizing selection
  2. Genetic drift
  3. Directional selection Correct answer.
  4. Disruptive selection

Explanation

Directional selection occurs when an environmental change shifts the population mean toward one extreme of a trait distribution. Here the average fur thickness moves in one direction across generations.

Question 39Natural sciences concepts

Which of the following best explains why a mutation in a single nucleotide can sometimes produce no change in the encoded protein?

  1. The mutation always occurs in a noncoding region
  2. Introns are removed before translation, so all mutations are harmless
  3. Ribosomes proofread mRNA and correct errors
  4. The genetic code is degenerate, so multiple codons specify the same amino acid Correct answer.

Explanation

Because the genetic code is redundant, a substitution can yield a synonymous codon that still specifies the same amino acid. Such silent mutations leave the protein sequence unchanged.

Question 40Natural sciences concepts

In a dihybrid cross of two heterozygotes (AaBb × AaBb) with independent assortment and complete dominance, what fraction of offspring is expected to show both dominant phenotypes?

  1. 9/16 Correct answer.
  2. 3/16
  3. 1/16
  4. 1/4

Explanation

Each gene independently gives a 3/4 chance of the dominant phenotype, so the joint probability is (3/4)(3/4) = 9/16. Independent assortment lets the probabilities multiply.

Question 41Natural sciences concepts

A block slides down a frictionless incline of height 5 m starting from rest. What is its speed at the bottom? (g = 10 m/s²)

  1. 5 m/s
  2. 20 m/s
  3. 50 m/s
  4. 10 m/s Correct answer.

Explanation

Conservation of energy gives mgh = ½mv², so v = √(2gh) = √(2 × 10 × 5) = 10 m/s. The mass cancels and the incline angle does not affect the final speed.

Question 42Natural sciences concepts

A 2 kg object moving at 6 m/s strikes a stationary 4 kg object, and the two stick together. What is the velocity of the combined object immediately after the collision?

  1. 4 m/s
  2. 6 m/s
  3. 2 m/s Correct answer.
  4. 3 m/s

Explanation

Momentum is conserved in a perfectly inelastic collision: (2 kg)(6 m/s) = (6 kg)v, giving v = 2 m/s. Kinetic energy is not conserved because the objects deform and stick.

Question 43Natural sciences concepts

Two resistors, 4 Ω and 6 Ω, are connected in parallel across a 12 V source. What is the total current drawn from the source?

  1. 1.2 A
  2. 2.0 A
  3. 5.0 A Correct answer.
  4. 3.0 A

Explanation

The equivalent resistance of the parallel pair is (4 × 6)/(4 + 6) = 2.4 Ω. From Ohm's law, I = V/R = 12/2.4 = 5.0 A.

Question 44Natural sciences concepts

A 0.5 kg mass on a spring with constant k = 50 N/m oscillates. What is the period of oscillation?

  1. 0.63 s Correct answer.
  2. 3.1 s
  3. 1.0 s
  4. 1.6 s

Explanation

For a mass–spring system, T = 2π√(m/k) = 2π√(0.5/50) = 2π(0.1) ≈ 0.63 s. Period depends on mass and stiffness, not amplitude.

Question 45Natural sciences concepts

A wave traveling along a string has a frequency of 200 Hz and a wavelength of 0.5 m. What is its speed?

  1. 50 m/s
  2. 400 m/s
  3. 200 m/s
  4. 100 m/s Correct answer.

Explanation

Wave speed equals frequency times wavelength: v = fλ = 200 Hz × 0.5 m = 100 m/s. Frequency and wavelength are related inversely through the fixed speed of the medium.

Question 46Natural sciences concepts

A 10 kg crate is pushed across a horizontal floor with a constant horizontal force of 50 N while kinetic friction of 20 N opposes the motion. What is the acceleration of the crate?

  1. 3 m/s² Correct answer.
  2. 5 m/s²
  3. 2 m/s²
  4. 7 m/s²

Explanation

The net horizontal force is 50 N − 20 N = 30 N. Using Newton's second law, a = Fnet/m = 30 N / 10 kg = 3 m/s².

Question 47Natural sciences concepts

An object is placed 20 cm in front of a concave mirror with a focal length of 10 cm. Where is the image formed?

  1. 20 cm behind the mirror
  2. 10 cm in front of the mirror
  3. 40 cm behind the mirror
  4. 20 cm in front of the mirror Correct answer.

Explanation

Using 1/f = 1/do + 1/di with f = 10 cm and do = 20 cm gives 1/di = 1/10 − 1/20 = 1/20, so di = 20 cm. A positive image distance for a mirror means a real image in front of the mirror.

Question 48Natural sciences concepts

A gas is compressed adiabatically and reversibly. Which statement about the gas is correct?

  1. Its temperature falls because heat leaves the system
  2. Its internal energy stays constant because Q = 0
  3. Its pressure decreases as volume decreases
  4. Its temperature rises because work is done on it with no heat exchange Correct answer.

Explanation

In an adiabatic process Q = 0, so ΔU = W. Compressing the gas means work is done on it, increasing internal energy and therefore temperature.

Question 49Natural sciences concepts

A sound source moves toward a stationary observer. How does the observed frequency compare with the emitted frequency?

  1. The observed frequency is unchanged
  2. The observed frequency is zero
  3. The observed frequency is higher Correct answer.
  4. The observed frequency is lower

Explanation

As the source approaches, successive wavefronts are compressed into a shorter distance, reducing the observed wavelength. Because wave speed in air is fixed, a shorter wavelength corresponds to a higher observed frequency.

Question 50Natural sciences concepts

A charged particle enters a uniform magnetic field moving parallel to the field lines. What is the magnetic force on the particle?

  1. Zero, because the velocity is parallel to the field Correct answer.
  2. Directed opposite to the velocity
  3. Maximum, because the charge is moving
  4. Perpendicular to both velocity and field

Explanation

The magnetic force is F = qvB sin θ, where θ is the angle between velocity and field. When the particle moves parallel to the field, θ = 0 and sin θ = 0, so no magnetic force acts.