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NPLEX I practice questions

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Question 1Part I – Biomedical Science

Which molecule is the immediate precursor for the synthesis of all steroid hormones in the adrenal cortex?

  1. Tyrosine
  2. Arachidonic acid
  3. Glucose
  4. Cholesterol Correct answer.

Explanation

Steroidogenesis begins with cholesterol, which is transported into mitochondria and cleaved by CYP11A1 (desmolase) to pregnenolone, the common precursor of all steroid hormones. The other listed molecules are precursors for eicosanoids, catecholamines, or energy substrates, not steroids.

Question 2Part I – Biomedical Science

A patient has a lesion of the left optic nerve anterior to the optic chiasm. Which visual field defect is expected?

  1. Right homonymous hemianopia
  2. Bitemporal hemianopia
  3. Total blindness of the left eye Correct answer.
  4. Left homonymous hemianopia

Explanation

Fibers from the nasal and temporal retina of one eye travel in that eye's optic nerve, so a prechiasmal lesion abolishes vision in the ipsilateral eye entirely. Chiasmal and retrochiasmal lesions produce the other patterns.

Question 3Part I – Biomedical Science

Which enzyme catalyzes the rate-limiting step of glycolysis?

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

Explanation

Phosphofructokinase-1 converts fructose-6-phosphate to fructose-1,6-bisphosphate and is the committed, rate-limiting step, regulated by ATP, AMP, and citrate. Hexokinase and pyruvate kinase are also regulated but do not set the overall glycolytic flux.

Question 4Part I – Biomedical Science

In the cardiac action potential, which conductance is chiefly responsible for the plateau phase?

  1. Rapid sodium current
  2. L-type calcium current Correct answer.
  3. Funny current
  4. Inward rectifier potassium current

Explanation

The plateau reflects a balance of depolarizing L-type calcium influx and repolarizing potassium efflux, with calcium entry sustaining phase 2. The rapid sodium current initiates phase 0, and the other currents dominate different phases.

Question 5Part I – Biomedical Science

A deficiency of vitamin B12 causes megaloblastic anemia primarily because of impaired activity of which enzyme?

  1. Pyruvate dehydrogenase
  2. Dihydrofolate reductase
  3. Thymidylate synthase
  4. Methionine synthase Correct answer.

Explanation

Vitamin B12 is a cofactor for methionine synthase, which regenerates tetrahydrofolate from methyl-THF; without it, folate is trapped and thymidylate synthesis fails, causing megaloblastosis. The other enzymes are not B12-dependent.

Question 6Part I – Biomedical Science

Which structure passes through the foramen ovale of the sphenoid bone?

  1. Facial nerve
  2. Mandibular division of the trigeminal nerve Correct answer.
  3. Maxillary division of the trigeminal nerve
  4. Ophthalmic division of the trigeminal nerve

Explanation

The foramen ovale transmits the mandibular division (V3) of the trigeminal nerve, along with the lesser petrosal nerve and accessory meningeal artery. The maxillary and ophthalmic divisions exit through different foramina, and the facial nerve exits the stylomastoid foramen.

Question 7Part I – Biomedical Science

Which immunoglobulin class is the first to be produced in a primary humoral immune response?

  1. IgG
  2. IgA
  3. IgM Correct answer.
  4. IgE

Explanation

Naive B cells express surface IgM and secrete IgM first after activation, before class switching produces other isotypes. IgG dominates secondary responses, and IgA and IgE appear after class switching.

Question 8Part I – Biomedical Science

A competitive inhibitor of an enzyme affects which kinetic parameter?

  1. Increases apparent Km with unchanged Vmax Correct answer.
  2. Decreases apparent Km with unchanged Vmax
  3. Decreases Vmax with unchanged Km
  4. Decreases both Km and Vmax

Explanation

A competitive inhibitor binds the active site reversibly, so it can be overcome by more substrate; Vmax is unchanged while apparent Km rises. Noncompetitive and uncompetitive inhibition change Vmax.

Question 9Part I – Biomedical Science

Which hormone is secreted by the corpus luteum and is essential for maintaining the early secretory endometrium?

  1. Luteinizing hormone
  2. Progesterone Correct answer.
  3. Follicle-stimulating hormone
  4. Estradiol

Explanation

After ovulation the corpus luteum secretes progesterone, which drives secretory transformation and supports early pregnancy until the placenta takes over. Estradiol peaks before ovulation, and the gonadotropins are pituitary hormones.

Question 10Part I – Biomedical Science

A patient with syndrome of inappropriate ADH secretion would be expected to show which laboratory pattern?

  1. Hyperkalemia with metabolic alkalosis
  2. Hypokalemia with metabolic acidosis
  3. Hyponatremia with concentrated urine Correct answer.
  4. Hypernatremia with dilute urine

Explanation

Excess ADH increases water reabsorption, diluting serum sodium while producing inappropriately concentrated urine. The other patterns reflect water loss or potassium and acid-base disturbances, not SIADH.

Question 11Part I – Biomedical Science

Which lung volumes or capacities cannot be measured by simple spirometry?

  1. Residual volume and total lung capacity Correct answer.
  2. Vital capacity and expiratory reserve volume
  3. Inspiratory capacity and tidal volume
  4. Tidal volume and inspiratory reserve volume

Explanation

Spirometry measures only air that moves in and out of the lungs, so residual volume and any capacity that includes it (such as total lung capacity) require plethysmography or gas dilution. The other listed volumes and capacities are directly measurable.

Question 12Part I – Biomedical Science

Which nerve is most likely injured in a fracture of the surgical neck of the humerus?

  1. Median nerve
  2. Radial nerve
  3. Axillary nerve Correct answer.
  4. Ulnar nerve

Explanation

The axillary nerve winds around the surgical neck of the humerus, so fractures there risk deltoid weakness and sensory loss over the lateral shoulder. The radial nerve is associated with the midshaft spiral groove, and the median and ulnar nerves are typically injured more distally.

Question 13Part I – Biomedical Science

Which of the following best describes the primary function of the sodium-potassium ATPase in most cells?

  1. Maintaining the resting membrane potential and cell volume Correct answer.
  2. Mediating neurotransmitter release
  3. Generating action potentials directly
  4. Driving calcium reuptake into the sarcoplasmic reticulum

Explanation

By extruding three sodium ions for two potassium ions, the pump maintains electrochemical gradients and osmotic balance, which underlie resting potential and cell volume. Action potentials arise from ion channels, and calcium reuptake uses a different pump.

Question 14Part I – Biomedical Science

A mutation that abolishes the function of the p53 gene would most directly promote which process?

  1. Defective DNA mismatch repair
  2. Uncontrolled cell cycle progression and genomic instability Correct answer.
  3. Failure of chromosomal segregation
  4. Increased apoptosis of damaged cells

Explanation

p53 is a tumor suppressor that arrests the cell cycle or triggers apoptosis after DNA damage; loss of function permits proliferation of damaged cells and accumulating mutations. Mismatch repair and mitotic segregation depend on other gene products.

Question 15Part I – Biomedical Science

Which phase of the cell cycle is characterized by DNA replication?

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

Explanation

DNA synthesis occurs during S phase, between the two gap phases. G1 is a growth phase, G2 prepares for mitosis, and M phase is where chromosome segregation and division occur.

Question 16Part I – Biomedical Science

A newborn presents with ambiguous genitalia and severe salt wasting. Which enzyme deficiency is most likely?

  1. 11-beta-hydroxylase deficiency
  2. 21-hydroxylase deficiency Correct answer.
  3. 17-alpha-hydroxylase deficiency
  4. Aromatase deficiency

Explanation

21-hydroxylase deficiency is the most common form of congenital adrenal hyperplasia; impaired cortisol and aldosterone synthesis causes salt wasting, while androgen excess virilizes female genitalia. The other blocks produce hypertension or different phenotypes.

Question 17Part I – Biomedical Science

Which of the following is the primary site of absorption of vitamin B12?

  1. Jejunum
  2. Stomach
  3. Duodenum
  4. Terminal ileum Correct answer.

Explanation

The B12-intrinsic factor complex binds cubilin receptors in the terminal ileum, where absorption occurs. The stomach produces intrinsic factor but does not absorb B12, and the more proximal small bowel absorbs other nutrients.

Question 18Part I – Biomedical Science

Which of the following best explains why a person with chronic renal failure develops anemia?

  1. Increased hepcidin causing iron overload
  2. Hemolysis from complement activation
  3. Reduced erythropoietin production Correct answer.
  4. Impaired intrinsic factor secretion

Explanation

The kidneys produce erythropoietin in response to hypoxia; loss of renal parenchyma reduces erythropoietin and therefore erythrocyte production. Iron overload, complement-mediated hemolysis, and intrinsic factor deficiency are not the primary mechanisms.

Question 19Part I – Biomedical Science

During the normal cardiac cycle, which event immediately follows the first heart sound?

  1. Closure of the semilunar valves Correct answer.
  2. Closure of the atrioventricular valves
  3. Opening of the atrioventricular valves
  4. Isovolumetric ventricular relaxation

Explanation

The first heart sound is produced by closure of the AV valves at the start of systole; ventricular pressure then rises, causing the semilunar valves to close later (the second sound). The remaining events occur in diastole.

Question 20Part I – Biomedical Science

Which of the following describes the inheritance pattern of an X-linked recessive disorder in a carrier mother?

  1. Half of her sons are affected and half of her daughters are carriers Correct answer.
  2. All of her sons are affected and all daughters are carriers
  3. All of her daughters are affected and half of her sons are carriers
  4. None of her sons are affected and half of her daughters are affected

Explanation

A carrier mother has one mutant and one normal X; each son has a 50% chance of inheriting the mutant allele and being affected, and each daughter has a 50% chance of being a carrier. Fathers contribute the other X to daughters, so daughters are rarely affected.

Question 21Part I – Biomedical Science

Which of the following is the primary neurotransmitter released by postganglionic sympathetic fibers innervating sweat glands?

  1. Acetylcholine Correct answer.
  2. Epinephrine
  3. Dopamine
  4. Norepinephrine

Explanation

Sweat glands are a classic exception to the sympathetic rule: their postganglionic fibers are cholinergic and release acetylcholine onto muscarinic receptors. Most other postganglionic sympathetic fibers release norepinephrine.

Question 22Part I – Biomedical Science

A deficiency of which vitamin impairs the gamma-carboxylation of clotting factors II, VII, IX, and X?

  1. Vitamin K Correct answer.
  2. Vitamin A
  3. Vitamin C
  4. Vitamin D

Explanation

Vitamin K is the cofactor for gamma-glutamyl carboxylase, which converts glutamate residues on these factors into calcium-binding gamma-carboxyglutamate. Deficiencies of the other vitamins affect collagen synthesis, vision, or calcium homeostasis.

Question 23Part I – Biomedical Science

A drug that blocks voltage-gated sodium channels in neurons would most directly affect which process?

  1. Axonal transport
  2. The rising phase of the action potential Correct answer.
  3. Resting membrane potential maintenance
  4. Neurotransmitter synthesis

Explanation

Voltage-gated sodium channels mediate the rapid depolarizing influx that produces the upstroke of the action potential; blockade slows or prevents this phase. Resting potential is maintained largely by potassium conductance and the sodium-potassium pump.

Question 24Part I – Biomedical Science

Which of the following best characterizes the role of the loop of Henle in urine concentration?

  1. It secretes hydrogen ions to acidify urine
  2. It establishes a medullary osmotic gradient through countercurrent multiplication Correct answer.
  3. It reabsorbs the majority of filtered glucose
  4. It filters plasma to form the ultrafiltrate

Explanation

The descending limb is water permeable and the ascending limb actively transports salt out while remaining water impermeable, generating a hypertonic medullary interstitium that permits water reabsorption in the collecting duct. The other functions belong to the proximal tubule or glomerulus.

Question 25Part I – Biomedical Science

Which of the following is a direct effect of parathyroid hormone on the kidney?

  1. Increased distal tubular calcium reabsorption Correct answer.
  2. Decreased activation of vitamin D
  3. Decreased phosphate excretion
  4. Increased sodium reabsorption

Explanation

PTH increases calcium reabsorption in the distal tubule while promoting phosphate excretion, and it stimulates renal 1-alpha-hydroxylase to activate vitamin D. The other choices contradict these actions.

Question 26Part I – Biomedical Science

Which type of collagen is the principal component of the basement membrane?

  1. Type III
  2. Type II
  3. Type IV Correct answer.
  4. Type I

Explanation

Type IV collagen forms a meshwork that is a defining structural element of basement membranes. It is produced largely by epithelial and endothelial cells.

Question 27Part I – Biomedical Science

In the fasting state, which process provides most of the glucose released by the liver?

  1. Hepatic gluconeogenesis Correct answer.
  2. Muscle glycolysis
  3. Hepatic glycogenolysis
  4. Adipose lipolysis

Explanation

After glycogen stores are depleted with prolonged fasting, gluconeogenesis in the liver becomes the principal source of blood glucose. Early fasting relies more on glycogenolysis, but sustained fasting is dominated by gluconeogenesis.

Question 28Part I – Biomedical Science

Which molecule serves as the final electron acceptor in the mitochondrial electron transport chain?

  1. Cytochrome c
  2. Oxygen Correct answer.
  3. NAD+
  4. FAD

Explanation

Complex IV transfers electrons to molecular oxygen, which is reduced to water. This keeps electrons flowing through the chain and sustains the proton gradient.

Question 29Part I – Biomedical Science

A patient has a deficiency of pyruvate dehydrogenase. Which metabolic consequence is most directly expected?

  1. Increased ketogenesis from acetyl-CoA
  2. Accumulation of acetyl-CoA
  3. Enhanced flux through the citric acid cycle
  4. Accumulation of pyruvate with increased lactate Correct answer.

Explanation

Pyruvate dehydrogenase links glycolysis to the citric acid cycle by converting pyruvate to acetyl-CoA. When it is deficient, pyruvate accumulates and is shunted to lactate via lactate dehydrogenase.

Question 30Part I – Biomedical Science

Which vitamin deficiency is most associated with megaloblastic anemia and neurologic symptoms including subacute combined degeneration?

  1. Vitamin C
  2. Vitamin B1
  3. Vitamin B12 Correct answer.
  4. Vitamin B3

Explanation

Vitamin B12 is required for methionine synthase and methylmalonyl-CoA mutase. Deficiency impairs DNA synthesis causing megaloblastic anemia and produces demyelination of the dorsal and lateral columns.

Question 31Part I – Biomedical Science

A competitive inhibitor raises the apparent Km of an enzyme without changing Vmax. Which statement best explains this finding?

  1. The inhibitor can be overcome by increasing substrate concentration Correct answer.
  2. The inhibitor decreases the catalytic constant of the enzyme
  3. The inhibitor binds irreversibly to the active site
  4. The inhibitor binds only to the enzyme-substrate complex

Explanation

A competitive inhibitor competes with substrate for the active site, so higher substrate concentrations can displace it and restore maximal velocity. This raises apparent Km while leaving Vmax unchanged.

Question 32Part I – Biomedical Science

Which component of the innate immune system directly lyses target cells by forming a membrane attack complex?

  1. Interferons
  2. Natural killer cells
  3. Acute-phase reactants
  4. Complement proteins Correct answer.

Explanation

The terminal complement components C5b through C9 assemble into a membrane attack complex that creates pores in target membranes, causing lysis.

Question 33Part I – Biomedical Science

A child has recurrent pyogenic infections, and laboratory testing shows defective oxidative burst in neutrophils. Which enzyme is most likely deficient?

  1. NADPH oxidase Correct answer.
  2. Glucose-6-phosphatase
  3. Adenosine deaminase
  4. Myeloperoxidase

Explanation

Chronic granulomatous disease results from defective NADPH oxidase, which prevents generation of superoxide and the respiratory burst needed to kill catalase-positive organisms.

Question 34Part I – Biomedical Science

Which immunoglobulin is the principal antibody in mucosal secretions?

  1. IgM
  2. IgE
  3. IgA Correct answer.
  4. IgG

Explanation

Secretory IgA is produced by plasma cells near mucosal surfaces and transported across epithelium. It is the dominant antibody class in saliva, tears, and gastrointestinal secretions.

Question 35Part I – Biomedical Science

During the cardiac cycle, closure of the semilunar valves produces which heart sound?

  1. First heart sound
  2. Fourth heart sound
  3. Second heart sound Correct answer.
  4. Third heart sound

Explanation

The second heart sound is generated by closure of the aortic and pulmonic valves at the end of ventricular ejection. It marks the onset of isovolumetric relaxation.

Question 36Part I – Biomedical Science

Which change in the pressure-volume loop results from an increase in arterial resistance (increased afterload)?

  1. Decreased end-systolic volume
  2. Increased stroke volume
  3. Increased end-systolic volume Correct answer.
  4. Decreased peak systolic pressure

Explanation

Higher afterload requires the ventricle to generate more pressure, and ejection is reduced, leaving more residual volume at end systole. End-systolic volume therefore rises.

Question 37Part I – Biomedical Science

Which factor is most responsible for the plateau phase of the ventricular myocyte action potential?

  1. Calcium influx through L-type channels Correct answer.
  2. Rapid sodium influx
  3. Potassium influx
  4. Chloride efflux

Explanation

The plateau (phase 2) results from a balance between slow inward calcium current through L-type channels and outward potassium current. This prolongs depolarization and supports contraction.

Question 38Part I – Biomedical Science

A patient has a lesion of the upper trunk of the brachial plexus. Which movement would most likely be impaired?

  1. Shoulder abduction and elbow flexion Correct answer.
  2. Finger flexion
  3. Wrist extension
  4. Hand grip strength

Explanation

The upper trunk gives rise to the musculocutaneous and axillary nerves, affecting the deltoid, supraspinatus, biceps, and brachialis. Shoulder abduction and elbow flexion are therefore impaired.

Question 39Part I – Biomedical Science

Which cranial nerve provides motor innervation to the muscles of mastication?

  1. Facial nerve
  2. Trigeminal nerve Correct answer.
  3. Glossopharyngeal nerve
  4. Hypoglossal nerve

Explanation

The mandibular division of the trigeminal nerve supplies the masseter, temporalis, and pterygoid muscles. These are derived from the first pharyngeal arch.

Question 40Part I – Biomedical Science

A patient has a lesion of the dorsal columns of the spinal cord. Which function is most directly lost?

  1. Autonomic regulation of blood pressure
  2. Voluntary motor control
  3. Fine touch and vibration sense Correct answer.
  4. Pain and temperature sensation

Explanation

The dorsal columns carry fine touch, vibration, and conscious proprioception that ascend to the medulla. Pain and temperature travel in the spinothalamic tracts.

Question 41Part I – Biomedical Science

Which neurotransmitter is released at the neuromuscular junction to trigger skeletal muscle contraction?

  1. Dopamine
  2. Norepinephrine
  3. Glutamate
  4. Acetylcholine Correct answer.

Explanation

Motor neurons release acetylcholine, which binds nicotinic receptors on the motor end plate to depolarize the muscle fiber. Acetylcholinesterase then terminates the signal.

Question 42Part I – Biomedical Science

Which hormone is released from the posterior pituitary?

  1. Adrenocorticotropic hormone
  2. Growth hormone
  3. Oxytocin Correct answer.
  4. Prolactin

Explanation

The posterior pituitary stores and releases oxytocin and vasopressin, which are synthesized in the hypothalamus. Prolactin, ACTH, and growth hormone come from the anterior pituitary.

Question 43Part I – Biomedical Science

A patient with primary hypothyroidism would be expected to show which laboratory pattern?

  1. High TSH and low free T4 Correct answer.
  2. Low TSH and low free T4
  3. Low TSH and high free T4
  4. High TSH and high free T4

Explanation

In primary hypothyroidism the thyroid gland fails to produce adequate hormone, removing negative feedback and causing elevated TSH. Free T4 is low because the gland cannot respond.

Question 44Part I – Biomedical Science

Which hormone is primarily responsible for the preovulatory surge that triggers ovulation?

  1. Follicle-stimulating hormone
  2. Progesterone
  3. Prolactin
  4. Luteinizing hormone Correct answer.

Explanation

Rising estradiol from the mature follicle exerts positive feedback on the hypothalamus and anterior pituitary, producing a midcycle LH surge. This surge triggers follicular rupture and ovulation.

Question 45Part I – Biomedical Science

Which renal structure is primarily responsible for the active reabsorption of the majority of filtered sodium and water?

  1. Collecting duct
  2. Glomerular capsule
  3. Proximal convoluted tubule Correct answer.
  4. Distal convoluted tubule

Explanation

The proximal tubule reabsorbs roughly two-thirds of filtered sodium and water through Na+/K+ ATPase-driven transport and aquaporin-mediated water movement. It is the principal site of bulk reabsorption.

Question 46Part I – Biomedical Science

Which acid-base disturbance is expected in a patient with severe prolonged vomiting?

  1. Respiratory alkalosis with low PCO2
  2. Metabolic alkalosis with high bicarbonate Correct answer.
  3. Metabolic acidosis with low bicarbonate
  4. Respiratory acidosis with high PCO2

Explanation

Vomiting removes gastric hydrochloric acid, causing loss of hydrogen and chloride. This produces a hypochloremic metabolic alkalosis with elevated serum bicarbonate.

Question 47Part I – Biomedical Science

In a population, a disease allele is present in carriers who do not express the trait, and affected individuals have two copies of the allele. Which inheritance pattern does this describe?

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

Explanation

Autosomal recessive disorders require two mutant alleles for expression, and heterozygous carriers remain unaffected. Carrier parents each contribute one mutant allele to affected offspring.

Question 48Part I – Biomedical Science

Which mechanism most commonly produces a frameshift mutation?

  1. Methylation of a cytosine residue
  2. Deletion of three consecutive nucleotides
  3. Substitution of one purine for another purine
  4. Insertion of a single nucleotide in a coding region Correct answer.

Explanation

Insertion or deletion of a number of nucleotides not divisible by three shifts the reading frame downstream. This alters all subsequent codons and usually produces a truncated, nonfunctional protein.

Question 49Part I – Biomedical Science

A competitive inhibitor and a noncompetitive inhibitor both reduce enzyme activity. Which kinetic pattern distinguishes the noncompetitive inhibitor?

  1. Vmax falls while apparent Km is unchanged Correct answer.
  2. Apparent Km falls while Vmax falls
  3. Apparent Km rises while Vmax is unchanged
  4. Both Vmax and apparent Km rise

Explanation

A pure noncompetitive inhibitor reduces the number of functional enzyme molecules, lowering Vmax without altering substrate affinity, so apparent Km is unchanged. Competitive inhibitors instead raise apparent Km with unchanged Vmax.

Question 50Part I – Biomedical Science

A patient has a mutation that prevents the synthesis of surfactant by type II pneumocytes. Which physiologic consequence is most directly expected?

  1. Increased alveolar surface tension with alveolar collapse Correct answer.
  2. Increased lung compliance
  3. Decreased work of breathing
  4. Increased diffusion capacity for oxygen

Explanation

Surfactant lowers alveolar surface tension and prevents collapse at end expiration according to Laplace's law. Without it, surface tension rises, alveoli tend to collapse, and compliance falls.