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Cambodia Health Entrance Exam study materials
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Cambodia Health Entrance Exam practice questions

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Question 1Biology

Which organelle is the main site of ATP synthesis in a human cell?

  1. Ribosome
  2. Lysosome
  3. Golgi apparatus
  4. Mitochondrion Correct answer.

Explanation

The mitochondrion houses the electron transport chain and ATP synthase on its inner membrane, so oxidative phosphorylation there produces most cellular ATP. The other listed organelles perform packaging, digestion, or protein synthesis rather than bulk ATP generation.

Question 2Biology

A patient's blood plasma is found to have a lower solute concentration than their red blood cells. What happens to the red blood cells when placed in this plasma?

  1. They shrink because water leaves the cells.
  2. They remain unchanged because the solutions are isotonic.
  3. They actively pump solutes out to equalize concentration.
  4. They swell and may lyse because water enters the cells. Correct answer.

Explanation

Water moves by osmosis from the region of lower solute concentration (the plasma) into the region of higher solute concentration (the cytoplasm). A hypotonic environment therefore causes red blood cells to gain water, swell, and potentially burst.

Question 3Biology

During which phase of mitosis do sister chromatids separate and move toward opposite poles of the cell?

  1. Telophase
  2. Prophase
  3. Anaphase Correct answer.
  4. Metaphase

Explanation

Anaphase begins when cohesin is cleaved and sister chromatids are pulled apart by spindle fibers toward opposite poles. Prophase involves condensation, metaphase alignment at the equator, and telophase nuclear reformation.

Question 4Biology

Which molecule serves as the immediate energy currency that powers muscle contraction and active transport in cells?

  1. Pyruvate
  2. ATP Correct answer.
  3. Glucose
  4. NADH

Explanation

ATP hydrolysis releases free energy that is directly coupled to endergonic processes such as myosin cross-bridge cycling and ion pumps. Glucose and pyruvate are fuel molecules that must first be oxidized to yield ATP.

Question 5Biology

In humans, which blood vessel carries oxygenated blood from the lungs to the heart?

  1. Aorta
  2. Pulmonary artery
  3. Pulmonary vein Correct answer.
  4. Superior vena cava

Explanation

The pulmonary veins are the exception to the rule that veins carry deoxygenated blood: they return oxygenated blood from the lungs to the left atrium. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs.

Question 6Biology

A person with blood group A receives a transfusion of group B red blood cells. What is the most likely consequence?

  1. No reaction, because both are ABO blood groups.
  2. Agglutination of recipient cells by the donor's anti-A antibodies.
  3. Immediate production of anti-A antibodies by the donor cells.
  4. Agglutination of donor cells by the recipient's anti-B antibodies. Correct answer.

Explanation

Group A plasma contains anti-B antibodies, which bind the B antigens on the transfused cells and cause agglutination and hemolysis. The donor's antibodies are diluted in the recipient's plasma and are not the primary danger in this scenario.

Question 7Biology

Which structure prevents food from entering the trachea during swallowing?

  1. Glottis
  2. Esophageal sphincter
  3. Epiglottis Correct answer.
  4. Uvula

Explanation

The epiglottis is a cartilaginous flap that folds over the glottis during swallowing, directing the bolus into the esophagus rather than the airway. The uvula and esophageal sphincter assist swallowing but do not guard the tracheal opening.

Question 8Biology

Which enzyme begins the chemical digestion of starch in the human mouth?

  1. Pepsin
  2. Salivary amylase Correct answer.
  3. Trypsin
  4. Lipase

Explanation

Salivary amylase hydrolyzes alpha-1,4 glycosidic bonds in starch, producing maltose and dextrins in the oral cavity. Pepsin and trypsin act on proteins in the stomach and small intestine, while lipase acts on lipids.

Question 9Biology

A mutation changes a codon from UAC to UAA in the middle of an mRNA sequence. What is the most likely effect on the polypeptide?

  1. A single amino acid substitution with no other change.
  2. A frameshift affecting all downstream codons.
  3. No change because the genetic code is degenerate.
  4. Premature termination of translation. Correct answer.

Explanation

UAA is one of the three stop codons, so the ribosome will terminate translation at that point and release a truncated polypeptide. A frameshift would require an insertion or deletion, not a substitution.

Question 10Biology

In a cross between two heterozygous individuals (Aa x Aa) for a trait showing complete dominance, what proportion of offspring is expected to show the recessive phenotype?

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

Explanation

The Punnett square gives genotype ratios of 1 AA : 2 Aa : 1 aa, so one quarter of the offspring are homozygous recessive and display the recessive phenotype. The dominant phenotype appears in the remaining three quarters.

Question 11Biology

A woman who is a carrier of an X-linked recessive disorder marries an unaffected man. What is the probability that a son will be affected?

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

Explanation

Sons inherit their single X chromosome from the mother, so a carrier mother passes the mutant allele to half her sons, who will all be affected because they lack a second X. Daughters are unaffected or carriers depending on which maternal X they inherit.

Question 12Biology

Which process produces genetic variation through the exchange of segments between homologous chromosomes?

  1. Crossing over Correct answer.
  2. DNA replication
  3. Cytokinesis
  4. Binary fission

Explanation

Crossing over occurs during prophase I of meiosis, when non-sister chromatids of homologous chromosomes exchange segments, creating new allele combinations. Replication copies DNA, cytokinesis divides the cytoplasm, and binary fission is prokaryotic reproduction.

Question 13Biology

Which hormone is secreted by the pancreas to lower blood glucose concentration after a meal?

  1. Glucagon
  2. Insulin Correct answer.
  3. Adrenaline
  4. Cortisol

Explanation

Insulin is released by pancreatic beta cells when blood glucose rises; it promotes cellular glucose uptake and glycogen synthesis, lowering blood glucose. Glucagon, cortisol, and adrenaline all act to raise blood glucose.

Question 14Biology

A patient is unable to produce antidiuretic hormone (ADH). Which change is most likely to occur?

  1. Increased water reabsorption in the collecting duct.
  2. Excretion of large volumes of dilute urine. Correct answer.
  3. Increased glucose reabsorption in the proximal tubule.
  4. Decreased sodium reabsorption in the proximal tubule.

Explanation

ADH increases the permeability of the collecting duct to water by inserting aquaporins, so its absence causes water to remain in the filtrate and be excreted as large volumes of dilute urine. Sodium and glucose handling in the proximal tubule is not directly controlled by ADH.

Question 15Biology

Which part of the nervous system controls involuntary functions such as heart rate and digestion?

  1. Somatic nervous system
  2. Sensory nervous system
  3. Autonomic nervous system Correct answer.
  4. Skeletal motor system

Explanation

The autonomic nervous system regulates smooth muscle, cardiac muscle, and glands, controlling involuntary processes such as heart rate and peristalsis. The somatic and skeletal motor systems control voluntary movement, and sensory pathways carry information inward.

Question 16Biology

After a nerve impulse reaches the axon terminal, which event directly triggers neurotransmitter release into the synaptic cleft?

  1. Opening of voltage-gated sodium channels in the terminal.
  2. Reuptake of neurotransmitter by the presynaptic neuron.
  3. Binding of neurotransmitter to postsynaptic receptors.
  4. Influx of calcium ions into the axon terminal. Correct answer.

Explanation

Depolarization of the axon terminal opens voltage-gated calcium channels, and the resulting calcium influx triggers fusion of synaptic vesicles with the membrane, releasing neurotransmitter. Sodium influx generates the action potential earlier, not vesicle fusion.

Question 17Biology

Which component of the immune system provides long-term protection by producing antibodies specific to a pathogen?

  1. Erythrocytes
  2. Neutrophils
  3. Macrophages
  4. B lymphocytes Correct answer.

Explanation

B lymphocytes differentiate into plasma cells that secrete antibodies specific to the antigen, and memory B cells provide long-term humoral immunity. Neutrophils and macrophages are phagocytes of innate immunity, and erythrocytes are red blood cells.

Question 18Biology

A child is vaccinated against measles and later is exposed to the measles virus. Which mechanism best explains the rapid protection?

  1. Memory B and T cells mount a faster, stronger secondary response. Correct answer.
  2. Innate inflammation clears the virus before symptoms appear.
  3. Antibiotics in the vaccine neutralize the virus.
  4. The vaccine virus remains active and kills the pathogen directly.

Explanation

Vaccination generates memory lymphocytes, so on re-exposure these cells proliferate and differentiate rapidly, producing high-affinity antibodies and cytotoxic activity before disease develops. This secondary response is faster and larger than the primary response.

Question 19Biology

In an ecosystem, which organisms convert atmospheric nitrogen into forms usable by plants?

  1. Nitrogen-fixing bacteria Correct answer.
  2. Decomposer fungi
  3. Denitrifying bacteria
  4. Primary consumers

Explanation

Nitrogen-fixing bacteria, such as Rhizobium in root nodules and free-living cyanobacteria, reduce atmospheric N2 to ammonia that plants can assimilate. Denitrifiers do the reverse, returning nitrogen to the atmosphere.

Question 20Biology

Which statement best describes the role of decomposers in a food web?

  1. They transfer energy directly to primary producers as food.
  2. They convert solar energy into chemical energy.
  3. They break down dead organic matter and recycle nutrients. Correct answer.
  4. They are always at the highest trophic level.

Explanation

Decomposers such as bacteria and fungi break down dead organisms and waste, releasing inorganic nutrients that producers can reuse. They operate across trophic levels rather than occupying a single high level.

Question 21Biology

Which human activity most directly increases the concentration of carbon dioxide in the atmosphere?

  1. Recycling aluminum
  2. Using solar panels
  3. Burning fossil fuels Correct answer.
  4. Planting forests

Explanation

Combustion of fossil fuels releases carbon that was stored underground into the atmosphere as CO2. Afforestation, solar energy, and recycling reduce or avoid emissions rather than adding CO2.

Question 22Biology

Two populations of the same bird species are separated by a mountain range for many generations. Which outcome is most consistent with speciation by geographic isolation?

  1. Accumulated genetic differences lead to reproductive isolation. Correct answer.
  2. Both populations immediately acquire identical mutations.
  3. The populations interbreed freely and remain genetically identical.
  4. Gene flow between the populations increases over time.

Explanation

Geographic isolation blocks gene flow, so mutation, genetic drift, and natural selection act independently in each population. Over many generations the accumulated differences can produce reproductive isolation, which defines separate species.

Question 23Biology

In a population at Hardy-Weinberg equilibrium, the frequency of the recessive allele is 0.2. What is the expected frequency of heterozygous individuals?

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

Explanation

If q = 0.2, then p = 0.8, and the heterozygote frequency is 2pq = 2(0.8)(0.2) = 0.32. The homozygous recessive frequency q squared is 0.04 and the homozygous dominant frequency p squared is 0.64.

Question 24Biology

A man with a normal hemoglobin genotype marries a woman who is a carrier of the sickle cell trait (heterozygous). What is the probability that a child will have sickle cell disease?

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

Explanation

Sickle cell disease requires two copies of the mutant beta-globin allele, but the father can contribute only a normal allele. Every child therefore inherits at least one normal allele and can be at most a carrier, so the probability of disease is zero.

Question 25Biology

An enzyme-catalyzed reaction is studied at increasing substrate concentrations. At high substrate concentration the reaction rate plateaus. What best explains this plateau?

  1. The activation energy of the reaction increases with substrate concentration.
  2. Substrate molecules begin to inhibit the enzyme competitively.
  3. The enzyme is permanently denatured at high substrate concentration.
  4. All active sites of the enzyme are saturated with substrate. Correct answer.

Explanation

When substrate concentration is high enough that essentially every enzyme active site is occupied, the enzyme is working at maximum velocity (Vmax) and adding more substrate cannot increase the rate. Saturation, not denaturation or a change in activation energy, produces the plateau.

Question 26Biology

Which molecule stores the genetic instructions that a human cell passes to its daughter cells during mitosis?

  1. Hemoglobin
  2. ATP
  3. Glucose
  4. DNA Correct answer.

Explanation

Deoxyribonucleic acid (DNA) carries the hereditary instructions in the nucleus and is copied before mitosis so each daughter cell receives a full set. ATP is an energy carrier, hemoglobin transports oxygen, and glucose is a fuel, so none of them stores genetic information.

Question 27Biology

A red blood cell is placed in distilled water. What will happen to the cell?

  1. It will stay the same size because water movement is equal.
  2. It will swell and may burst because water enters by osmosis. Correct answer.
  3. It will dissolve completely in the water.
  4. It will shrink because water leaves the cell.

Explanation

Distilled water has a lower solute concentration than the cytoplasm, so it is hypotonic to the cell. Water moves inward by osmosis across the cell membrane, causing the cell to swell, and a red blood cell without a rigid wall may burst (hemolysis).

Question 28Biology

Which organelle is the main site of aerobic ATP production in a human liver cell?

  1. Lysosome
  2. Ribosome
  3. Golgi apparatus
  4. Mitochondrion Correct answer.

Explanation

Mitochondria host the electron transport chain and oxidative phosphorylation, which generate most ATP during aerobic respiration. Ribosomes assemble proteins, the Golgi apparatus modifies and packages them, and lysosomes digest material.

Question 29Biology

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

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

Explanation

DNA replication takes place during the S (synthesis) phase of interphase, producing two identical sister chromatids for each chromosome. G1 and G2 are growth phases, and M phase is when the duplicated chromosomes are separated and the cell divides.

Question 30Biology

Which blood vessel carries oxygenated blood from the lungs to the heart?

  1. Pulmonary artery
  2. Pulmonary vein Correct answer.
  3. Aorta
  4. Superior vena cava

Explanation

The pulmonary vein returns oxygenated blood from the lungs to the left atrium, a key exception to the rule that veins carry deoxygenated blood. The pulmonary artery carries deoxygenated blood to the lungs, the vena cava returns deoxygenated blood from the body, and the aorta carries blood away from the left ventricle.

Question 31Biology

Which process in the human kidney removes waste such as urea from the blood into the tubule?

  1. Secretion of glucose
  2. Reabsorption in the loop of Henle
  3. Production of red blood cells
  4. Filtration at the glomerulus Correct answer.

Explanation

Blood is filtered under pressure at the glomerulus, and small molecules including urea, water, glucose, and ions pass into the tubule while cells and large proteins remain in the blood. Reabsorption returns useful substances to the blood, and red blood cell production occurs in bone marrow.

Question 32Biology

Which type of muscle is found in the walls of the stomach and intestines?

  1. Striated voluntary muscle
  2. Smooth muscle Correct answer.
  3. Cardiac muscle
  4. Skeletal muscle

Explanation

Smooth muscle forms the walls of hollow internal organs such as the stomach and intestines and contracts involuntarily to move contents along the digestive tract. Skeletal muscle is voluntary and attached to bones, while cardiac muscle is confined to the heart.

Question 33Biology

A student observes that a plant kept in the dark for several days loses its green color. Which pigment is being lost?

  1. Carotene
  2. Melanin
  3. Chlorophyll Correct answer.
  4. Hemoglobin

Explanation

Chlorophyll is the green pigment that absorbs light for photosynthesis and is broken down when light is absent, causing leaves to appear pale or yellow. Melanin is an animal pigment, hemoglobin carries oxygen in blood, and carotene is an accessory pigment that is not the main green color.

Question 34Biology

In a dihybrid cross between two individuals heterozygous for both traits (AaBb x AaBb), what phenotypic ratio is expected if the genes assort independently and show complete dominance?

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

Explanation

Independent assortment of two genes each with a 3:1 dominant-to-recessive ratio gives a combined ratio of 9:3:3:1 for the four phenotype combinations. The 3:1 ratio applies to a monohybrid cross, and 1:2:1 describes a genotypic ratio for one gene.

Question 35Biology

A woman with blood group AB and a man with blood group O have a child. Which blood groups are possible in the child?

  1. AB or O
  2. O only
  3. A or B Correct answer.
  4. A, B, AB, or O

Explanation

The mother with AB has genotype IAIB and passes either IA or IB, while the father with O has genotype ii and passes only i. The child is therefore IAi (group A) or IBi (group B), so only groups A and B are possible.

Question 36Biology

Which change to a DNA sequence is most likely to cause a frameshift mutation?

  1. Methylation of a cytosine base
  2. Insertion of two bases Correct answer.
  3. Substitution of one base for another
  4. Deletion of three bases

Explanation

Inserting or deleting a number of bases that is not a multiple of three shifts the reading frame, so every downstream codon is read differently. A three-base deletion removes one whole codon and leaves the frame intact, while substitution and methylation do not shift the frame.

Question 37Biology

Which structure in a flowering plant produces the male gametes?

  1. Anther Correct answer.
  2. Stigma
  3. Ovule
  4. Sepal

Explanation

The anther contains pollen sacs where meiosis produces pollen grains, and each pollen grain develops the male gametes. The ovule holds the female egg cell, the stigma receives pollen, and sepals protect the developing flower bud.

Question 38Biology

A man has a mutation that prevents the production of testosterone by his testes. Which effect is most directly expected?

  1. Loss of sperm production and reduced development of male secondary sexual characteristics Correct answer.
  2. Failure of the kidneys to filter blood
  3. Inability to digest proteins in the stomach
  4. Immediate loss of all red blood cells

Explanation

Testosterone from the interstitial cells of the testes supports spermatogenesis and the development of male secondary sexual characteristics such as facial hair and a deep voice. It does not control red blood cell survival, stomach protein digestion, or kidney filtration.

Question 39Biology

Which sequence correctly describes the pathway of a nerve impulse through a reflex arc?

  1. Motor neuron, sensory neuron, interneuron, receptor, effector
  2. Receptor, motor neuron, interneuron, sensory neuron, effector
  3. Effector, interneuron, receptor, motor neuron, sensory neuron
  4. Receptor, sensory neuron, interneuron, motor neuron, effector Correct answer.

Explanation

A reflex arc begins with a receptor detecting a stimulus and sending impulses along a sensory neuron to the spinal cord. There an interneuron relays the signal to a motor neuron, which carries it to an effector such as a muscle that produces the response.

Question 40Biology

Which feature of the alveoli most directly increases the efficiency of gas exchange in the lungs?

  1. A large total surface area with thin walls Correct answer.
  2. A lining of ciliated epithelial cells
  3. A thick layer of smooth muscle in their walls
  4. A rich supply of cartilage rings

Explanation

Millions of alveoli provide a very large total surface area, and their walls are only one cell thick, so oxygen and carbon dioxide diffuse rapidly across a short distance. Cartilage and thick muscle would impede diffusion rather than aid it.

Question 41Biology

In an ecosystem, energy is transferred from producers to primary consumers. Approximately what proportion of the energy in producers is typically transferred to the next trophic level?

  1. About 10 percent Correct answer.
  2. About 1 percent
  3. About 50 percent
  4. About 90 percent

Explanation

Roughly 10 percent of the energy at one trophic level is passed to the next, with the rest lost mainly as heat through respiration and in excreted material. This low transfer efficiency limits food chains to about four or five levels.

Question 42Biology

Which interaction between two species is best described as mutualism?

  1. A tick feeding on the blood of a dog
  2. A lion preying on a zebra
  3. Nitrogen-fixing bacteria living in root nodules of a legume Correct answer.
  4. A tapeworm absorbing nutrients inside a human intestine

Explanation

In mutualism both species benefit: the bacteria receive carbohydrates and shelter from the plant, while the plant gains usable nitrogen compounds. Parasitism benefits one species at the expense of the other, and predation benefits only the predator.

Question 43Biology

A bacterial population is exposed to an antibiotic, and a few cells survive and multiply. Which statement best explains this outcome according to natural selection?

  1. Some bacteria already carried resistance genes and survived to reproduce. Correct answer.
  2. The surviving bacteria learned to resist the antibiotic during exposure.
  3. The antibiotic caused resistant mutations in every bacterial cell.
  4. The antibiotic became weaker after the first treatment.

Explanation

Natural selection acts on variation that already exists: cells carrying resistance genes survive the antibiotic and pass those genes to their offspring, so the population becomes resistant. Antibiotics select resistant cells but do not create the mutations, and bacteria do not learn resistance.

Question 44Biology

In a population at Hardy-Weinberg equilibrium, the frequency of the dominant allele A is 0.7. What is the expected frequency of heterozygous individuals?

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

Explanation

If p = 0.7, then q = 0.3, and the heterozygote frequency is 2pq = 2 x 0.7 x 0.3 = 0.42. The value 0.49 is p squared, and 0.09 is q squared, which are the homozygous frequencies.

Question 45Biology

Which human activity contributes most directly to the greenhouse effect by releasing carbon stored in fossil fuels?

  1. Burning coal in a power station Correct answer.
  2. Building a dam for hydroelectric power
  3. Planting trees in a deforested area
  4. Using wind turbines to generate electricity

Explanation

Burning coal releases carbon dioxide that was locked in fossil deposits for millions of years, adding new greenhouse gas to the atmosphere. Planting trees, hydropower, and wind power either remove carbon dioxide or avoid adding fossil carbon.

Question 46Biology

A hospital isolates a bacterium from a wound and finds it is a Gram-negative rod that ferments lactose. Which laboratory feature would most help identify it as Escherichia coli?

  1. Requirement for high salt concentrations to survive
  2. Production of indole from tryptophan Correct answer.
  3. Formation of endospores under aerobic conditions
  4. Ability to grow only at 4 degrees Celsius

Explanation

Escherichia coli is a lactose-fermenting Gram-negative rod that produces the enzyme tryptophanase, converting tryptophan to indole, a positive indole test. It does not form endospores, does not require high salt, and grows well at body temperature rather than only in the cold.

Question 47Biology

A patient with severe diarrhea loses a large volume of fluid and electrolytes. Which compensatory change is most likely in the kidneys?

  1. Increased filtration of glucose into the urine
  2. Decreased reabsorption of water in the collecting duct
  3. Increased reabsorption of water and sodium to conserve blood volume Correct answer.
  4. Reduced secretion of renin by the juxtaglomerular cells

Explanation

Fluid loss lowers blood volume and pressure, so the kidneys conserve water and sodium through increased reabsorption, partly driven by aldosterone and antidiuretic hormone. This helps restore blood volume and maintain blood pressure.

Question 48Biology

A mutation in a gene encoding a cell membrane receptor makes the receptor active even when no hormone is bound. Which consequence is most likely?

  1. The hormone is no longer produced by the endocrine gland.
  2. The target cell stops responding to the hormone completely.
  3. The target cell shows continuous responses even at low hormone levels. Correct answer.
  4. The receptor is rapidly removed from the membrane and destroyed.

Explanation

A constitutively active receptor sends signals without ligand binding, so downstream responses occur continuously even when hormone concentration is low. This differs from loss-of-function mutations, which reduce responsiveness.

Question 49Biology

In a pedigree, a rare disorder appears in both males and females, affected children can have unaffected parents, and the trait does not skip generations after appearing. Which inheritance pattern is most consistent with these observations?

  1. Autosomal recessive Correct answer.
  2. Mitochondrial
  3. Y-linked
  4. X-linked dominant

Explanation

Autosomal recessive disorders can appear in children of unaffected carrier parents, affect both sexes, and seem to skip generations. X-linked dominant and Y-linked traits show characteristic sex patterns, and mitochondrial traits pass through mothers.

Question 50Biology

An enzyme has a mutation that changes an amino acid in its active site, reducing its affinity for the substrate. How will this most likely affect the Michaelis constant (Km) and maximum velocity (Vmax)?

  1. Km decreases and Vmax remains unchanged
  2. Km remains unchanged and Vmax decreases
  3. Km increases and Vmax may remain unchanged Correct answer.
  4. Km decreases and Vmax increases

Explanation

Km is inversely related to substrate affinity, so weaker binding raises Km, meaning more substrate is needed to reach half of Vmax. If the catalytic machinery is intact, Vmax can stay the same; only if catalysis is also impaired would Vmax fall.