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ECOM-CR Medical Licensure study materials
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ECOM-CR Medical Licensure practice questions

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Question 1Basic Medical Sciences

A 24-year-old man sustains a midshaft humeral fracture. Which nerve is most likely injured because it runs in the spiral groove against the humerus?

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

Explanation

The radial nerve descends in the spiral (radial) groove of the humerus, where it is tightly applied to bone and vulnerable in midshaft fractures. Injury produces wrist drop and sensory loss over the posterior forearm and dorsum of the hand.

Question 2Basic Medical Sciences

Which heart valve is best heard at the left fifth intercostal space at the midclavicular line?

  1. Mitral valve Correct answer.
  2. Aortic valve
  3. Pulmonary valve
  4. Tricuspid valve

Explanation

The mitral valve is auscultated at the cardiac apex, which lies in the left fifth intercostal space at the midclavicular line. This apical location reflects transmission of sound from the mitral orifice toward the chest wall.

Question 3Basic Medical Sciences

During the cardiac cycle, which event coincides with closure of the semilunar valves and marks the onset of the second heart sound?

  1. End of ventricular systole Correct answer.
  2. Start of atrial contraction
  3. Peak of the QRS complex
  4. Opening of the atrioventricular valves

Explanation

The second heart sound is produced when the aortic and pulmonary valves close at the end of ventricular systole. This marks the boundary between systole and diastole, occurring just after the T wave.

Question 4Basic Medical Sciences

A patient's blood gas shows pH 7.30, PaCO2 55 mmHg, and bicarbonate 26 mmol/L. Which acid-base disturbance is present?

  1. Metabolic alkalosis
  2. Respiratory acidosis Correct answer.
  3. Respiratory alkalosis
  4. Metabolic acidosis

Explanation

The low pH indicates acidemia, and the elevated PaCO2 identifies a respiratory cause. A normal bicarbonate with acute CO2 retention is consistent with acute respiratory acidosis before renal compensation develops.

Question 5Basic Medical Sciences

Which segment of the nephron is the primary site of action of aldosterone on sodium reabsorption?

  1. Glomerular capillary endothelium
  2. Descending limb of the loop of Henle
  3. Proximal convoluted tubule
  4. Distal convoluted tubule and collecting duct Correct answer.

Explanation

Aldosterone acts on principal cells of the late distal tubule and collecting duct, increasing sodium reabsorption and potassium secretion. It upregulates epithelial sodium channels and the Na-K ATPase in these segments.

Question 6Basic Medical Sciences

Which immunoglobulin class is the first to be produced in a primary humoral response and is most abundant in serum?

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

Explanation

IgM is the first antibody produced in a primary immune response, and it is also the most abundant immunoglobulin in serum as a pentamer. Its multivalency makes it an efficient activator of complement.

Question 7Basic Medical Sciences

Which brain structure is the principal relay station for sensory pathways traveling to the cerebral cortex?

  1. Cerebellum
  2. Basal ganglia
  3. Hypothalamus
  4. Thalamus Correct answer.

Explanation

The thalamus relays sensory information from the body and special senses to the cerebral cortex, except for olfaction. Each sensory modality projects through specific thalamic nuclei.

Question 8Basic Medical Sciences

Which vitamin deficiency classically produces night blindness due to impaired rhodopsin formation?

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

Explanation

Vitamin A provides retinal, which combines with opsin to form rhodopsin in rod photoreceptors. Deficiency impairs dark adaptation and causes night blindness, and later can cause xerophthalmia.

Question 9Basic Medical Sciences

A 60-year-old man has crushing substernal chest pain and ST elevation in leads II, III, and aVF. Which coronary artery is most likely occluded?

  1. Right coronary artery Correct answer.
  2. Left main coronary artery
  3. Left circumflex artery
  4. Left anterior descending artery

Explanation

ST elevation in the inferior leads II, III, and aVF localizes the infarct to the inferior wall, most often supplied by the right coronary artery. The right coronary artery also supplies the AV node in most people.

Question 10Basic Medical Sciences

A patient with severe vomiting develops hypokalemia and metabolic alkalosis. Which renal mechanism most directly contributes to the hypokalemia?

  1. Reduced aldosterone secretion
  2. Increased distal sodium delivery driving potassium secretion Correct answer.
  3. Increased proximal phosphate reabsorption
  4. Decreased tubular fluid flow

Explanation

Vomiting causes volume depletion and aldosterone excess, while increased distal sodium delivery promotes sodium reabsorption in exchange for potassium secretion. This renal potassium wasting worsens hypokalemia.

Question 11Basic Medical Sciences

Which type of hypersensitivity reaction underlies acute hemolytic transfusion reactions from ABO incompatibility?

  1. Type I immediate hypersensitivity
  2. Type III immune complex hypersensitivity
  3. Type IV cell-mediated hypersensitivity
  4. Type II antibody-mediated hypersensitivity Correct answer.

Explanation

Preformed antibodies bind ABO antigens on donor red cells, causing complement activation and intravascular hemolysis. This antibody-mediated destruction of cell-surface antigens defines a type II reaction.

Question 12Basic Medical Sciences

A 45-year-old woman has fatigue and pallor, with a peripheral smear showing microcytic hypochromic red cells and a low ferritin. Which finding best supports iron deficiency anemia?

  1. Increased ringed sideroblasts
  2. Low total iron-binding capacity
  3. Elevated serum ferritin
  4. Elevated total iron-binding capacity Correct answer.

Explanation

In iron deficiency, low iron stores reduce ferritin and raise transferrin, so total iron-binding capacity rises. The resulting low transferrin saturation produces microcytic hypochromic anemia.

Question 13Basic Medical Sciences

Which phase of the cell cycle is the primary target of the spindle assembly checkpoint?

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

Explanation

The spindle assembly checkpoint operates during mitosis, delaying anaphase until every chromosome is properly attached to spindle microtubules. This prevents unequal chromosome segregation.

Question 14Basic Medical Sciences

A 30-year-old woman reports heat intolerance, weight loss, and tremor, and has a diffuse goiter with elevated free T4 and suppressed TSH. Which mechanism best explains these findings?

  1. Destruction of thyroid tissue
  2. Dietary iodine deficiency
  3. Pituitary adenoma secreting TSH
  4. Autoantibodies stimulating the TSH receptor Correct answer.

Explanation

Suppressed TSH with elevated free T4 indicates a primary hyperthyroid state, and diffuse goiter points to Graves disease. Stimulating TSH-receptor antibodies drive both hormone overproduction and gland enlargement.

Question 15Basic Medical Sciences

Which lung volume cannot be measured by simple spirometry and requires body plethysmography or helium dilution?

  1. Residual volume Correct answer.
  2. Tidal volume
  3. Inspiratory reserve volume
  4. Vital capacity

Explanation

Residual volume is the air remaining after maximal expiration and cannot be exhaled into a spirometer. It is derived indirectly, for example by plethysmography or gas dilution, to calculate total lung capacity.

Question 16Basic Medical Sciences

A patient has metabolic acidosis with a normal anion gap and a history of chronic diarrhea. Which process best explains the acidosis?

  1. Increased renal hydrogen ion excretion
  2. Lactic acid overproduction
  3. Bicarbonate loss from the gastrointestinal tract Correct answer.
  4. Accumulation of ketone bodies

Explanation

Chronic diarrhea causes direct loss of bicarbonate-rich fluid, producing a hyperchloremic normal-anion-gap acidosis. The kidneys retain chloride as bicarbonate is lost, so the anion gap remains normal.

Question 17Basic Medical Sciences

Which enzyme catalyzes the rate-limiting step of glycolysis?

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

Explanation

Phosphofructokinase-1 catalyzes the committed, rate-limiting step of glycolysis by phosphorylating fructose-6-phosphate. It is regulated by ATP, citrate, AMP, and fructose-2,6-bisphosphate.

Question 18Basic Medical Sciences

A 55-year-old man has resting tremor, bradykinesia, and rigidity. Which neurotransmitter deficiency best accounts for these findings?

  1. Serotonin in the raphe nuclei
  2. Dopamine in the substantia nigra Correct answer.
  3. Acetylcholine in the basal ganglia
  4. GABA in the cerebellum

Explanation

Parkinson disease results from degeneration of dopaminergic neurons in the substantia nigra pars compacta. Loss of striatal dopamine produces the classic triad of tremor, rigidity, and bradykinesia.

Question 19Basic Medical Sciences

Which renal tubular segment is impermeable to water and actively reabsorbs sodium via the Na-K-2Cl cotransporter?

  1. Thick ascending limb of the loop of Henle Correct answer.
  2. Descending limb of the loop of Henle
  3. Medullary collecting duct
  4. Proximal convoluted tubule

Explanation

The thick ascending limb actively transports sodium, potassium, and chloride with the Na-K-2Cl cotransporter and is impermeable to water. This dilutes tubular fluid and contributes to the medullary concentration gradient.

Question 20Basic Medical Sciences

A tumor suppressor gene requires loss of both alleles to produce disease, consistent with Knudson's hypothesis. Which gene is the classic example in retinoblastoma?

  1. RB1 Correct answer.
  2. RAS
  3. MYC
  4. BCR-ABL

Explanation

Knudson's two-hit hypothesis describes tumor suppressors such as RB1, where both alleles must be inactivated. Hereditary retinoblastoma inherits one defective RB1 allele, so a single somatic hit in retinal cells suffices.

Question 21Basic Medical Sciences

A 28-year-old woman has a butterfly rash, photosensitivity, and arthralgia. Which serologic finding is most specific for systemic lupus erythematosus?

  1. Anti-histone antibody
  2. Anti-double-stranded DNA antibody Correct answer.
  3. Rheumatoid factor
  4. Antinuclear antibody

Explanation

Anti-double-stranded DNA antibodies are highly specific for systemic lupus erythematosus and correlate with disease activity and lupus nephritis. Antinuclear antibody is sensitive but not specific.

Question 22Basic Medical Sciences

A patient develops flushing, diarrhea, and bronchospasm from a carcinoid tumor. Which mediator most directly accounts for these features?

  1. Serotonin Correct answer.
  2. Cortisol
  3. Insulin
  4. Parathyroid hormone

Explanation

Carcinoid tumors secrete serotonin and other vasoactive amines that cause flushing, diarrhea, and bronchospasm. Serotonin also drives endocardial fibrosis in carcinoid heart disease.

Question 23Basic Medical Sciences

A 70-year-old smoker has hemoptysis and a central lung mass with squamous differentiation. Which paraneoplastic syndrome is most characteristic of this histologic type?

  1. Hyponatremia from ADH excess
  2. Cushing syndrome from ectopic ACTH
  3. Hypercalcemia from PTH-related peptide Correct answer.
  4. Hypoglycemia from insulin-like growth factor

Explanation

Squamous cell carcinoma of the lung commonly secretes parathyroid hormone-related peptide, producing humoral hypercalcemia of malignancy. Small cell carcinoma is more often associated with ectopic ACTH and ADH.

Question 24Basic Medical Sciences

A patient with chronic kidney disease has an anion gap of 22 mmol/L and a bicarbonate of 12 mmol/L. Using the delta-gap approach, which interpretation is most appropriate?

  1. Pure respiratory alkalosis
  2. High-anion-gap acidosis with a concurrent metabolic alkalosis
  3. Pure high-anion-gap acidosis with no additional disorder
  4. High-anion-gap acidosis with a concurrent normal-anion-gap acidosis Correct answer.

Explanation

The delta gap compares the rise in anion gap with the fall in bicarbonate; here the gap rises by about 10 while bicarbonate falls by about 12, so the change in bicarbonate exceeds what the anion gap alone explains. That indicates an additional normal-anion-gap acidosis, as from diarrhea or renal tubular acidosis.

Question 25Basic Medical Sciences

A 62-year-old man has progressive dysphagia to solids and weight loss, and a barium study shows a distal esophageal mass. Which histologic type predominates in this location in Western populations?

  1. Squamous cell carcinoma
  2. Small cell carcinoma
  3. Leiomyosarcoma
  4. Adenocarcinoma Correct answer.

Explanation

Esophageal adenocarcinoma arises from Barrett metaplasia in the distal esophagus and is now the predominant type in Western countries. Squamous cell carcinoma more often involves the upper and middle esophagus and is linked to smoking and alcohol.

Question 26Basic Medical Sciences

Which component of the sarcomere shortens during skeletal muscle contraction according to the sliding filament theory?

  1. The I band Correct answer.
  2. The A band
  3. The Z disc width
  4. The thick filament length

Explanation

During contraction, actin filaments slide past myosin, so the I band and H zone narrow while the A band stays constant because thick filament length does not change. The Z discs are pulled closer together rather than widening.

Question 27Basic Medical Sciences

Which molecule serves as the immediate energy source for the power stroke of the myosin head during cross-bridge cycling?

  1. Calcium ions
  2. ATP Correct answer.
  3. Creatine phosphate
  4. Acetylcholine

Explanation

ATP binding to myosin causes detachment from actin, and hydrolysis of ATP energizes the myosin head for the next power stroke. Creatine phosphate is a reserve that regenerates ATP but is not used directly by the cross-bridge.

Question 28Basic Medical Sciences

A patient has a lesion destroying the right optic tract. Which visual field defect is expected?

  1. Bitemporal hemianopia
  2. Right homonymous hemianopia
  3. Right monocular blindness
  4. Left homonymous hemianopia Correct answer.

Explanation

The right optic tract carries fibers from the temporal retina of the right eye and nasal retina of the left eye, both of which subserve the left visual field. Destruction therefore produces a left homonymous hemianopia.

Question 29Basic Medical Sciences

Which structure produces cerebrospinal fluid within the ventricular system?

  1. Pia mater
  2. Arachnoid granulations
  3. Ependymal cilia
  4. Choroid plexus Correct answer.

Explanation

The choroid plexus, located in the lateral, third, and fourth ventricles, secretes cerebrospinal fluid. Arachnoid granulations resorb CSF into venous sinuses rather than producing it.

Question 30Basic Medical Sciences

Which hormone is released from the posterior pituitary?

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

Explanation

Oxytocin and antidiuretic hormone are synthesized in hypothalamic neurons and stored in and released from the posterior pituitary. The other listed hormones are anterior pituitary products.

Question 31Basic Medical Sciences

A patient with type 1 diabetes develops ketoacidosis. Which hormonal state most directly drives the excessive lipolysis and ketogenesis?

  1. Elevated insulin with low glucagon
  2. Low insulin with elevated glucagon Correct answer.
  3. Low insulin with low glucagon
  4. Elevated insulin with elevated glucagon

Explanation

In diabetic ketoacidosis, insulin deficiency removes inhibition of hormone-sensitive lipase while glucagon excess promotes fatty acid oxidation and ketone body formation in the liver. The combination of low insulin and high glucagon is the key driver.

Question 32Basic Medical Sciences

Which renal process is responsible for the glucose reabsorption threshold observed at approximately 180 mg/dL plasma concentration?

  1. Inability of the loop of Henle to dilute urine
  2. Overload of the organic anion secretory pathway
  3. Failure of ADH-dependent water reabsorption
  4. Saturation of SGLT2 and SGLT1 cotransporters in the proximal tubule Correct answer.

Explanation

Glucose is reabsorbed by sodium-glucose cotransporters in the proximal tubule, and these carriers reach maximal transport capacity at a plasma glucose near 180 mg/dL. Once saturated, the excess glucose remains in the filtrate and appears in urine.

Question 33Basic Medical Sciences

Which acid-base disturbance is expected in a patient with prolonged vomiting from gastric outlet obstruction?

  1. Metabolic alkalosis with hypochloremia Correct answer.
  2. Metabolic acidosis with increased anion gap
  3. Respiratory alkalosis with hypocapnia
  4. Respiratory acidosis with hypercapnia

Explanation

Loss of gastric hydrochloric acid removes chloride and hydrogen ions, generating a hypochloremic metabolic alkalosis with low urine chloride. Volume depletion also drives aldosterone-mediated sodium retention with bicarbonate retention.

Question 34Basic Medical Sciences

Which enzyme deficiency causes the accumulation of galactocerebroside and is characteristic of Krabbe disease?

  1. Galactocerebrosidase Correct answer.
  2. Glucocerebrosidase
  3. Sphingomyelinase
  4. Hexosaminidase A

Explanation

Krabbe disease results from deficient galactocerebrosidase, leading to accumulation of galactocerebroside and psychosine in oligodendrocytes and Schwann cells. This causes demyelination and globoid cells on histology.

Question 35Basic Medical Sciences

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

  1. Excess ketone production with hypoglycemia
  2. Defective fatty acid beta-oxidation
  3. Impaired gluconeogenesis with lactic acidosis Correct answer.
  4. Impaired urea cycle with hyperammonemia

Explanation

Pyruvate carboxylase converts pyruvate to oxaloacetate, a key gluconeogenic precursor, so its deficiency impairs gluconeogenesis and shunts pyruvate to lactate. This produces lactic acidosis, particularly during fasting or illness.

Question 36Basic Medical Sciences

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

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

Explanation

Type IV collagen forms the network scaffold of basement membranes and is a major component of the basal lamina. Types I, II, and III are interstitial collagens found in bone, cartilage, and extensible tissues respectively.

Question 37Basic Medical Sciences

Which mechanism best explains the increased risk of infection in a patient with chronic granulomatous disease?

  1. Impaired neutrophil oxidative burst Correct answer.
  2. Defective T-cell receptor signaling
  3. Absent B-cell immunoglobulin production
  4. Complement C3 deficiency

Explanation

Chronic granulomatous disease results from defective NADPH oxidase, so phagocytes cannot generate reactive oxygen species to kill catalase-positive organisms. This produces recurrent infections with organisms such as Staphylococcus aureus and Aspergillus.

Question 38Basic Medical Sciences

Which feature distinguishes Hodgkin lymphoma from non-Hodgkin lymphoma on histologic examination?

  1. Presence of a Philadelphia chromosome
  2. Uniform population of small lymphocytes
  3. Reed-Sternberg cells in an inflammatory background Correct answer.
  4. Absence of B-cell markers

Explanation

Classic Hodgkin lymphoma contains binucleate Reed-Sternberg cells with prominent nucleoli in a background of reactive inflammatory cells. Non-Hodgkin lymphomas typically show a more uniform neoplastic lymphoid population.

Question 39Basic Medical Sciences

A 55-year-old man has a lung mass and elevated parathyroid hormone-related peptide. Which electrolyte abnormality is most likely?

  1. Hyperkalemia
  2. Hypophosphatemia
  3. Hypernatremia
  4. Hypercalcemia Correct answer.

Explanation

PTHrP from squamous cell lung carcinoma mimics parathyroid hormone, increasing bone resorption and renal calcium reabsorption to produce hypercalcemia. This is the classic humoral hypercalcemia of malignancy.

Question 40Basic Medical Sciences

Which structural feature of the lung is primarily responsible for the large surface area available for gas exchange?

  1. Ciliated epithelium of the trachea
  2. Smooth muscle in the terminal bronchioles
  3. Cartilage rings in the bronchi
  4. Alveolar septa with extensive capillary networks Correct answer.

Explanation

Gas exchange occurs across the thin alveolar-capillary membrane, and the vast number of alveoli provides the large surface area. Cartilage, smooth muscle, and ciliated epithelium serve airway support and clearance rather than diffusion.

Question 41Basic Medical Sciences

Which change in pulmonary function is expected in a patient with pulmonary fibrosis?

  1. Increased airway resistance with normal diffusion capacity
  2. Increased compliance with reduced elastic recoil
  3. Normal compliance with increased residual volume
  4. Reduced compliance with decreased total lung capacity Correct answer.

Explanation

Pulmonary fibrosis stiffens the lung parenchyma, lowering compliance and reducing total lung capacity, producing a restrictive pattern. Diffusion capacity is also typically reduced.

Question 42Basic Medical Sciences

Which factor most directly increases oxygen unloading from hemoglobin in exercising muscle?

  1. Increased pH
  2. Decreased temperature
  3. Increased 2,3-BPG and decreased pH Correct answer.
  4. Decreased carbon dioxide

Explanation

The Bohr effect shifts the oxygen dissociation curve to the right when pH falls and 2,3-BPG rises, reducing hemoglobin affinity and enhancing oxygen delivery to tissues. Exercising muscle generates CO2 and acid, promoting this shift.

Question 43Basic Medical Sciences

Which component of the cardiac conduction system normally sets the resting heart rate because it has the fastest spontaneous depolarization?

  1. Purkinje fibers
  2. Atrioventricular node
  3. Bundle of His
  4. Sinoatrial node Correct answer.

Explanation

The sinoatrial node has the steepest phase 4 spontaneous depolarization and therefore the highest intrinsic rate, so it normally dominates as the pacemaker. Other pacemakers fire more slowly and are overdrive-suppressed.

Question 44Basic Medical Sciences

Which vascular change primarily mediates the increased cardiac output during early exercise?

  1. Systemic vasoconstriction with increased afterload
  2. Reduced heart rate with prolonged diastole
  3. Decreased sympathetic activity to the heart
  4. Venoconstriction with increased venous return Correct answer.

Explanation

Sympathetic venoconstriction increases venous return, which raises preload and cardiac output through the Frank-Starling mechanism. Sympathetic activity also increases heart rate and contractility.

Question 45Basic Medical Sciences

Which mechanism explains the peripheral edema seen in nephrotic syndrome?

  1. Increased capillary permeability from histamine
  2. Lymphatic obstruction by tumor
  3. Reduced plasma oncotic pressure from hypoalbuminemia Correct answer.
  4. Increased capillary hydrostatic pressure from heart failure

Explanation

Heavy urinary protein loss lowers serum albumin, reducing plasma oncotic pressure and allowing fluid to move into the interstitium. Sodium retention also contributes to the edema.

Question 46Basic Medical Sciences

Which phase of hemostasis is directly impaired by a deficiency of von Willebrand factor?

  1. Secondary hemostasis via fibrin cross-linking
  2. Primary hemostasis via platelet adhesion Correct answer.
  3. Fibrinolysis via plasmin activation
  4. Vasoconstriction via endothelin release

Explanation

Von Willebrand factor mediates platelet adhesion to exposed subendothelial collagen, so deficiency impairs primary hemostasis and produces mucocutaneous bleeding. It also carries factor VIII, but the platelet adhesion defect is the defining abnormality.

Question 47Basic Medical Sciences

Which feature of the bacterial cell wall is targeted by penicillin?

  1. Lipopolysaccharide in the outer membrane
  2. Peptidoglycan cross-linking by transpeptidases Correct answer.
  3. Bacterial 70S ribosome
  4. DNA gyrase

Explanation

Penicillins are beta-lactams that acylate penicillin-binding proteins, blocking the transpeptidase cross-linking of peptidoglycan and weakening the cell wall. This leads to osmotic lysis in actively dividing bacteria.

Question 48Basic Medical Sciences

A patient with a wound infection grows Staphylococcus aureus that is resistant to methicillin. Which mechanism most commonly accounts for this resistance?

  1. Porin loss in the outer membrane
  2. Efflux pump overexpression
  3. Production of an altered penicillin-binding protein Correct answer.
  4. Ribosomal methylation

Explanation

MRSA resistance is mediated by the mecA gene encoding PBP2a, an altered penicillin-binding protein with low affinity for beta-lactams. This allows continued peptidoglycan cross-linking despite drug exposure.

Question 49Basic Medical Sciences

Which type of vaccine contains an inactivated bacterial toxin that has been chemically modified to retain immunogenicity?

  1. Toxoid vaccine Correct answer.
  2. Live attenuated vaccine
  3. mRNA vaccine
  4. Conjugate polysaccharide vaccine

Explanation

A toxoid is a bacterial exotoxin treated with formalin or heat to eliminate toxicity while preserving antigenic epitopes, as in tetanus and diphtheria vaccines. It induces antitoxin antibodies rather than infection-derived immunity.

Question 50Basic Medical Sciences

Which mechanism best explains the genetic basis of Down syndrome in most affected individuals?

  1. Balanced translocation between chromosomes 14 and 21
  2. Uniparental disomy of chromosome 21
  3. Nondisjunction during meiosis producing trisomy 21 Correct answer.
  4. Deletion of the long arm of chromosome 21

Explanation

Most Down syndrome cases result from meiotic nondisjunction, usually maternal, producing a free trisomy 21. Robertsonian translocation accounts for a small minority of cases.