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JMC Human Medicine Exam study materials
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JMC Human Medicine Exam practice questions

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Question 1Anatomy

Which nerve passes through the carpal tunnel together with the flexor tendons?

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

Explanation

The carpal tunnel is bounded by the flexor retinaculum and the carpal bones, and its contents are the median nerve and the flexor tendons of the fingers and thumb. The ulnar nerve passes superficial to the retinaculum through Guyon's canal, not through the tunnel.

Question 2Anatomy

A patient cannot abduct the arm beyond about 15 degrees and has a laterally rotated scapula on testing. Which muscle is most likely paralysed?

  1. Deltoid
  2. Supraspinatus Correct answer.
  3. Subscapularis
  4. Teres minor

Explanation

The supraspinatus initiates the first 15 degrees of abduction before the deltoid takes over, so isolated loss of early abduction points to a suprascapular nerve lesion affecting supraspinatus. Deltoid paralysis would impair abduction throughout the range rather than only its initiation.

Question 3Anatomy

Which structure forms the medial boundary of the femoral triangle?

  1. Adductor longus muscle Correct answer.
  2. Sartorius muscle
  3. Inguinal ligament
  4. Pectineus muscle

Explanation

The femoral triangle is bounded laterally by sartorius, medially by the medial border of adductor longus, and superiorly by the inguinal ligament, with the floor formed by iliopsoas and pectineus. Adductor longus therefore defines the medial side.

Question 4Anatomy

Occlusion of the artery of Adamkiewicz during aortic surgery most threatens which spinal cord region?

  1. Lower thoracic and lumbar segments Correct answer.
  2. Sacral conus only
  3. Cervical enlargement
  4. Upper thoracic segments

Explanation

The artery of Adamkiewicz is the major radiculomedullary feeder, usually arising from a lower intercostal or lumbar artery, and supplies the lower thoracic and lumbar cord. Its compromise produces anterior spinal artery syndrome with paraplegia and sphincter disturbance.

Question 5Anatomy

Which cranial nerve carries preganglionic parasympathetic fibres to the parotid gland?

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

Explanation

Preganglionic parasympathetic fibres for the parotid travel in the glossopharyngeal nerve, pass via the lesser petrosal nerve to the otic ganglion, and relay to the gland through the auriculotemporal nerve. The facial nerve supplies the other major salivary glands.

Question 6Anatomy

A midshaft humeral fracture injures the nerve lying in the spiral groove. Which deficit is expected?

  1. Loss of elbow flexion
  2. Loss of wrist extension with wrist drop Correct answer.
  3. Loss of thumb opposition
  4. Loss of sensation over the little finger

Explanation

The radial nerve runs in the spiral groove of the humerus, and injury there paralyses the wrist extensors, producing wrist drop with sensory loss over the dorsum of the hand. Thumb opposition and little-finger sensation are median and ulnar functions respectively.

Question 7Anatomy

Which chamber of the heart forms the greater part of the anterior surface?

  1. Right ventricle Correct answer.
  2. Right atrium
  3. Left ventricle
  4. Left atrium

Explanation

The right ventricle forms most of the anterior surface of the heart and the inferior border, because the heart is rotated so that the right ventricle faces forward. The left ventricle forms the apex and much of the posterior and left border.

Question 8Anatomy

Which coronary artery supplies the atrioventricular node in most people?

  1. Anterior interventricular artery
  2. Right coronary artery Correct answer.
  3. Circumflex artery
  4. Left marginal artery

Explanation

In the majority of individuals the atrioventricular node is supplied by a branch of the right coronary artery, which also supplies the sinoatrial node in about 60 percent of people. The anterior interventricular artery chiefly supplies the anterior ventricular walls and septum.

Question 9Anatomy

Which feature distinguishes the right main bronchus from the left?

  1. It is longer and more horizontal
  2. It arises above the carina
  3. It lacks cartilaginous rings
  4. It is wider, shorter, and more vertical Correct answer.

Explanation

The right main bronchus is wider, shorter, and more vertical than the left, which explains the greater frequency of inhaled foreign bodies lodging on the right side. The left bronchus is longer and more horizontal because it must pass lateral to the aortic arch.

Question 10Anatomy

Which muscle is the chief inspiratory muscle that lowers the diaphragm during quiet breathing?

  1. Sternocleidomastoid
  2. External intercostal muscle
  3. Diaphragm Correct answer.
  4. Rectus abdominis

Explanation

The diaphragm is the principal muscle of inspiration; contraction flattens it and increases the vertical dimension of the thoracic cavity. The external intercostals assist by elevating the ribs, and the sternocleidomastoid is an accessory inspiratory muscle.

Question 11Anatomy

A lesion of the left recurrent laryngeal nerve after thyroid surgery most typically causes which finding?

  1. Hoarseness from impaired movement of the left vocal fold Correct answer.
  2. Loss of taste from the anterior tongue
  3. Paralysis of the left half of the soft palate
  4. Anaesthesia of the left external ear

Explanation

The left recurrent laryngeal nerve supplies all intrinsic laryngeal muscles except cricothyroid, so its injury weakens the left vocal fold and produces hoarseness. The nerve is vulnerable during thyroidectomy because it ascends in the tracheo-oesophageal groove.

Question 12Anatomy

Which structure lies immediately anterior to the oesophagus as it descends through the superior mediastinum?

  1. Trachea Correct answer.
  2. Azygos vein
  3. Descending aorta
  4. Thoracic duct

Explanation

In the superior mediastinum the oesophagus lies directly behind the trachea, which explains the spread of oesophageal tumours into the airway. The descending aorta and azygos vein lie posterior or lateral to the oesophagus.

Question 13Anatomy

Which part of the nephron is the principal site of regulated water reabsorption under antidiuretic hormone?

  1. Bowman's capsule
  2. Proximal convoluted tubule
  3. Collecting duct Correct answer.
  4. Descending limb of the loop of Henle

Explanation

Antidiuretic hormone inserts aquaporin-2 channels in the collecting duct epithelium, allowing regulated water reabsorption under the medullary osmotic gradient. The proximal tubule reabsorbs most filtered water but does so constitutively, not under hormonal control.

Question 14Anatomy

Which ligament prevents anterior displacement of the tibia on the femur during knee flexion?

  1. Medial collateral ligament
  2. Posterior cruciate ligament
  3. Anterior cruciate ligament Correct answer.
  4. Fibular collateral ligament

Explanation

The anterior cruciate ligament resists anterior translation of the tibia relative to the femur and limits hyperextension, which is why the anterior drawer test assesses it. The posterior cruciate ligament resists the opposite displacement.

Question 15Anatomy

A patient has foot drop and loss of sensation over the dorsum of the foot after a fibular neck injury. Which nerve is affected?

  1. Sural nerve
  2. Obturator nerve
  3. Tibial nerve
  4. Common fibular nerve Correct answer.

Explanation

The common fibular nerve winds around the fibular neck, where it is superficial and easily injured, and supplies the dorsiflexors and evertors with sensation to the dorsum of the foot. Injury produces foot drop and a steppage gait.

Question 16Anatomy

Which structure passes through the greater sciatic foramen above piriformis?

  1. Sciatic nerve
  2. Pudendal nerve
  3. Superior gluteal nerve Correct answer.
  4. Posterior cutaneous nerve of thigh

Explanation

The superior gluteal nerve and vessels leave the pelvis through the greater sciatic foramen above piriformis, whereas the sciatic nerve and most other structures pass below it. This distinction underlies piriformis syndrome.

Question 17Anatomy

Which endocrine gland lies in the sella turcica of the sphenoid bone?

  1. Adrenal gland
  2. Thyroid gland
  3. Pituitary gland Correct answer.
  4. Pineal gland

Explanation

The pituitary gland occupies the hypophyseal fossa of the sella turcica in the sphenoid bone, connected to the hypothalamus by the infundibulum. The pineal gland lies on the roof of the third ventricle.

Question 18Anatomy

Which vessel is the usual site of anastomosis in a portocaval shunt for portal hypertension?

  1. Hepatic veins to inferior vena cava
  2. Left gastric vein to oesophageal veins
  3. Splenic vein to left renal vein Correct answer.
  4. Superior mesenteric vein to inferior vena cava

Explanation

The classic splenorenal shunt joins the splenic vein to the left renal vein, decompressing the portal system through the renal vein into the inferior vena cava. Portosystemic anastomoses also occur at the oesophagus and rectum but are sites of varices rather than surgical shunts.

Question 19Anatomy

Which layer of the meninges contains the arachnoid granulations that return cerebrospinal fluid to the venous system?

  1. Dura mater
  2. Pia mater
  3. Periosteum
  4. Arachnoid mater Correct answer.

Explanation

Arachnoid granulations project from the arachnoid mater into the superior sagittal sinus and transfer cerebrospinal fluid into the venous blood. The pia mater closely invests the brain surface, and the dura is the outer fibrous layer.

Question 20Anatomy

A lesion of the lateral corticospinal tract in the cervical cord produces which pattern of weakness?

  1. Contralateral flaccid weakness at the level of the lesion
  2. Ipsilateral spastic weakness below the lesion Correct answer.
  3. Ipsilateral loss of vibration sense below the lesion
  4. Bilateral loss of pain and temperature sensation

Explanation

Corticospinal fibres cross in the medulla, so a lateral corticospinal lesion in the cervical cord causes ipsilateral spastic weakness below the level of the lesion. The dorsal columns carry vibration sense and the spinothalamic tract carries pain and temperature.

Question 21Physiology

During the upstroke of the ventricular myocyte action potential, which ion movement is mainly responsible for the rapid depolarisation?

  1. Influx of chloride through ligand-gated channels
  2. Influx of calcium through L-type channels
  3. Rapid influx of sodium through voltage-gated channels Correct answer.
  4. Efflux of potassium through delayed rectifier channels

Explanation

Phase 0 of the ventricular action potential is produced by opening of fast voltage-gated sodium channels, and the resulting large inward sodium current drives the membrane potential rapidly towards positive values. Calcium entry is slower and contributes mainly to the plateau, while potassium efflux drives repolarisation.

Question 22Physiology

Which change in the lung most increases the diffusing capacity for oxygen during strenuous exercise?

  1. Reduction of alveolar surface tension by surfactant
  2. Increase in anatomic dead space
  3. Recruitment of previously closed pulmonary capillaries Correct answer.
  4. Shortening of the diffusion path by alveolar fluid accumulation

Explanation

Diffusing capacity depends on surface area and on the thickness of the alveolar-capillary membrane. Exercise raises pulmonary blood flow and pressure, which opens and recruits pulmonary capillaries, increasing the effective surface area available for gas exchange. Dead space enlargement would not help, and fluid accumulation thickens the membrane.

Question 23Physiology

A patient has a plasma osmolality of 305 mOsm/kg with a serum sodium of 150 mmol/L. Which hormonal response is most directly expected?

  1. Increased secretion of atrial natriuretic peptide
  2. Increased release of antidiuretic hormone from the posterior pituitary Correct answer.
  3. Decreased renin release by the juxtaglomerular apparatus
  4. Increased secretion of parathyroid hormone

Explanation

Plasma osmolality above the normal range of roughly 275-295 mOsm/kg is detected by hypothalamic osmoreceptors, which trigger antidiuretic hormone release and thirst. ADH then increases water reabsorption in the collecting duct to dilute the plasma. Volume-regulating hormones such as ANP respond mainly to volume rather than osmolality.

Question 24Physiology

Which structure is the main site of reabsorption of filtered glucose and amino acids in the nephron?

  1. Medullary collecting duct
  2. Proximal convoluted tubule Correct answer.
  3. Thin descending limb of the loop of Henle
  4. Distal convoluted tubule

Explanation

The proximal convoluted tubule reabsorbs the bulk of filtered glucose and amino acids via sodium-coupled cotransport, supported by its dense apical brush border and abundant mitochondria. More distal segments handle fine-tuning of sodium, water and acid-base balance rather than bulk organic solute recovery.

Question 25Physiology

In the normal cardiac cycle, which event coincides with closure of the aortic valve?

  1. The beginning of the second heart sound Correct answer.
  2. The peak of the QRS complex on the ECG
  3. The opening of the mitral valve
  4. The beginning of the first heart sound

Explanation

The second heart sound is produced by closure of the semilunar valves, with the aortic component occurring at the end of ventricular ejection. The first heart sound marks closure of the atrioventricular valves at the start of systole.

Question 26Physiology

Which factor most increases the affinity of haemoglobin for oxygen?

  1. A rise in blood pH Correct answer.
  2. A rise in temperature
  3. An increase in 2,3-diphosphoglycerate
  4. An increase in carbon dioxide partial pressure

Explanation

The oxygen-haemoglobin dissociation curve shifts left when pH rises, reflecting the Bohr effect. Higher pH reduces the tendency of haemoglobin to release oxygen, so affinity increases. Warmth, 2,3-DPG and carbon dioxide all shift the curve right and favour oxygen unloading.

Question 27Physiology

Which lung volume cannot be measured directly by simple spirometry?

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

Explanation

Residual volume is the air remaining in the lungs after a maximal expiration and cannot be exhaled into a spirometer. It is therefore obtained indirectly, for example by helium dilution, nitrogen washout or body plethysmography. Tidal and reserve volumes are all directly measurable.

Question 28Physiology

A rise in arterial partial pressure of carbon dioxide from 40 to 46 mmHg produces which immediate change in ventilation?

  1. A decrease caused by peripheral chemoreceptor inhibition
  2. An increase driven mainly by central chemoreceptors Correct answer.
  3. A decrease mediated by pulmonary stretch receptors
  4. No change because oxygen is unchanged

Explanation

Carbon dioxide crosses the blood-brain barrier and generates hydrogen ions in cerebrospinal fluid, stimulating central chemoreceptors, which are the dominant drive to ventilation. The response to a rise in carbon dioxide is brisk and roughly linear over the physiological range.

Question 29Physiology

Which statement best describes the role of the sarcoplasmic reticulum in skeletal muscle contraction?

  1. It provides the elastic recoil that lengthens the sarcomere
  2. It stores calcium and releases it after excitation of the T-tubule membrane Correct answer.
  3. It generates ATP by oxidative phosphorylation during contraction
  4. It binds acetylcholine released at the motor end plate

Explanation

Depolarisation spreads along the T-tubules and triggers calcium release from the sarcoplasmic reticulum, allowing calcium to bind troponin C and expose actin binding sites. Reuptake of calcium into the sarcoplasmic reticulum then permits relaxation.

Question 30Physiology

According to the sliding filament theory, what happens to the A band during muscle shortening?

  1. It lengthens as thin filaments slide outward
  2. It remains constant in length Correct answer.
  3. It shortens in proportion to the sarcomere
  4. It disappears when the muscle is fully contracted

Explanation

The A band corresponds to the thick filament, whose length does not change during contraction. Shortening occurs because thin filaments slide past thick filaments, which narrows the I band and the H zone while the A band stays the same.

Question 31Physiology

Which hormone is released from the adrenal cortex in response to angiotensin II and raises sodium reabsorption?

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

Explanation

Angiotensin II stimulates the zona glomerulosa of the adrenal cortex to secrete aldosterone, which increases sodium reabsorption and potassium secretion in the distal nephron. This expands extracellular fluid volume as part of the renin-angiotensin-aldosterone axis.

Question 32Physiology

Which process in the gastrointestinal tract is primarily responsible for absorption of long-chain fatty acids?

  1. Formation of micelles followed by diffusion into enterocytes and chylomicron assembly Correct answer.
  2. Simple diffusion of intact triglycerides into portal capillaries
  3. Cotransport of fatty acids with sodium at the apical membrane
  4. Active transport of free fatty acids across the basolateral membrane

Explanation

Bile salts emulsify fat and form micelles that deliver fatty acids and monoglycerides to the brush border. These diffuse into enterocytes, are re-esterified into triglycerides, packaged into chylomicrons and secreted into lacteals rather than portal blood.

Question 33Physiology

Which change occurs in the sinoatrial node during strong vagal stimulation?

  1. Increased sodium influx through funny channels
  2. Shortening of the action potential duration
  3. Increased potassium conductance and reduced slope of the pacemaker potential Correct answer.
  4. Increased calcium influx through T-type channels

Explanation

Acetylcholine acting on muscarinic receptors opens potassium channels and reduces the funny current and calcium currents in the sinoatrial node. The resulting hyperpolarisation and slower diastolic depolarisation reduce the heart rate.

Question 34Physiology

A patient with a lesion destroying the posterior pituitary develops excessive dilute urine. Which hormone is deficient?

  1. Thyroxine
  2. Atrial natriuretic peptide
  3. Aldosterone
  4. Antidiuretic hormone Correct answer.

Explanation

Antidiuretic hormone is stored and released from the posterior pituitary and increases water permeability in the collecting duct. Its deficiency causes diabetes insipidus with large volumes of dilute urine. Aldosterone and ANP regulate sodium balance rather than water reabsorption directly.

Question 35Physiology

Which mechanism best explains the increased stroke volume produced by a rise in end-diastolic volume?

  1. Increased heart rate reducing filling time
  2. Enhanced parasympathetic tone to the ventricles
  3. Reduced afterload from systemic vasoconstriction
  4. Increased overlap of actin and myosin cross-bridges optimising force generation Correct answer.

Explanation

Greater end-diastolic volume stretches the ventricular wall, which improves actin-myosin overlap and increases the force of contraction according to the Frank-Starling relationship. The result is a larger stroke volume for the same afterload.

Question 36Physiology

Which statement correctly describes the countercurrent multiplier system of the loop of Henle?

  1. Active sodium chloride reabsorption in the thick ascending limb creates a medullary osmotic gradient Correct answer.
  2. The descending limb actively pumps sodium into the interstitium
  3. Water reabsorption in the ascending limb concentrates the tubular fluid
  4. The vasa recta generate the gradient by secreting urea

Explanation

The thick ascending limb actively transports sodium chloride into the interstitium while remaining relatively impermeable to water, which dilutes the tubular fluid and builds a hyperosmotic medulla. The descending limb then allows water to leave, concentrating the fluid entering the ascending limb.

Question 37Physiology

Which receptor mediates the rapid increase in heart rate produced by circulating adrenaline?

  1. Beta-1 adrenergic receptors on the sinoatrial node Correct answer.
  2. Muscarinic M2 receptors on the sinoatrial node
  3. Beta-2 adrenergic receptors on bronchial smooth muscle
  4. Alpha-1 adrenergic receptors on vascular smooth muscle

Explanation

Adrenaline binds beta-1 adrenergic receptors on the sinoatrial node, raising the slope of the pacemaker potential and increasing heart rate. Alpha-1 receptors mediate vasoconstriction and M2 receptors slow the heart, while beta-2 receptors mainly relax bronchial muscle.

Question 38Physiology

Which factor most determines the resting membrane potential of a typical neuron?

  1. The intracellular concentration of calcium
  2. The activity of the sodium-potassium pump independent of ion gradients
  3. The sodium concentration gradient alone
  4. The potassium concentration gradient and membrane permeability to potassium Correct answer.

Explanation

At rest the neuronal membrane is far more permeable to potassium than to sodium, so the membrane potential lies close to the potassium equilibrium potential. The sodium-potassium pump maintains the gradients that sustain this potential.

Question 39Physiology

Which change in the alveolus increases the tendency for alveolar collapse according to Laplace's law?

  1. A decrease in alveolar radius at constant surface tension Correct answer.
  2. An increase in alveolar wall thickness
  3. An increase in alveolar radius at constant surface tension
  4. A decrease in surface tension by surfactant

Explanation

Laplace's law states that the collapsing pressure of a sphere is proportional to surface tension and inversely proportional to radius. A smaller alveolus therefore generates a higher collapsing pressure, which is why surfactant lowers surface tension more in small alveoli to keep them stable.

Question 40Physiology

Which component of a reflex arc carries the signal from the sensory receptor towards the central nervous system?

  1. Afferent neuron Correct answer.
  2. Efferent neuron
  3. Interneuron
  4. Motor end plate

Explanation

The afferent neuron conducts impulses from the sensory receptor into the central nervous system, where interneurons integrate the signal. The efferent neuron then carries the response to the effector.

Question 41Biochemistry

Which enzyme catalyzes the committed, rate-limiting step of glycolysis?

  1. Hexokinase
  2. Phosphofructokinase-1 Correct answer.
  3. Glucose-6-phosphatase
  4. Pyruvate kinase

Explanation

Phosphofructokinase-1 converts fructose-6-phosphate to fructose-1,6-bisphosphate and is the main rate-limiting, committed step of glycolysis, regulated by ATP, citrate, AMP and fructose-2,6-bisphosphate. Hexokinase and pyruvate kinase also catalyze irreversible steps but are not the committed regulatory step. Glucose-6-phosphatase runs in the opposite direction during gluconeogenesis.

Question 42Biochemistry

In the citric acid cycle, which reaction produces GTP or ATP by substrate-level phosphorylation?

  1. Citrate to isocitrate
  2. Fumarate to malate
  3. Isocitrate to alpha-ketoglutarate
  4. Succinyl-CoA to succinate Correct answer.

Explanation

Succinyl-CoA synthetase converts succinyl-CoA to succinate, coupling cleavage of the thioester to phosphorylation of GDP or ADP, yielding GTP or ATP. The other listed steps are oxidative decarboxylations or isomerizations that do not generate high-energy phosphate directly.

Question 43Biochemistry

A patient's muscle biopsy shows impaired fatty acid oxidation. Carnitine palmitoyltransferase I (CPT1) is required for which process?

  1. Beta-oxidation of short-chain fatty acids inside peroxisomes
  2. Transport of long-chain fatty acyl-CoA into the mitochondrial matrix Correct answer.
  3. Conversion of acetyl-CoA to ketone bodies in the liver
  4. Activation of fatty acids to acyl-CoA in the cytosol

Explanation

CPT1 on the outer mitochondrial membrane converts long-chain acyl-CoA to acylcarnitine, allowing passage across the inner membrane via the carnitine shuttle. This step is rate-limiting for mitochondrial beta-oxidation. Short-chain fatty acids can enter without CPT1, and ketogenesis is a separate hepatic pathway.

Question 44Biochemistry

Which amino acid is the precursor for serotonin (5-hydroxytryptamine)?

  1. Tryptophan Correct answer.
  2. Tyrosine
  3. Glutamate
  4. Histidine

Explanation

Tryptophan is hydroxylated by tryptophan hydroxylase and then decarboxylated to serotonin. Tyrosine gives rise to catecholamines and thyroid hormone, histidine to histamine, and glutamate to GABA.

Question 45Biochemistry

A newborn presents with severe hypoglycemia, hepatomegaly, and lactic acidosis; enzyme testing confirms glucose-6-phosphatase deficiency (von Gierke disease). Which biochemical finding is expected?

  1. Elevated blood lactate with poor glucose response to glucagon Correct answer.
  2. Normal blood lactate with fasting ketosis only
  3. Elevated blood lactate with massive ketonemia and no hepatomegaly
  4. Low blood lactate with a strong hyperglycemic response to glucagon

Explanation

In glucose-6-phosphatase deficiency, glucose-6-phosphate cannot be dephosphorylated to free glucose, so it is shunted into glycolysis, raising lactate. Glucagon cannot raise glucose because the final release step is blocked. Ketosis tends to be modest because lipolysis is suppressed by the lactate and glucose-6-phosphate excess.

Question 46Biochemistry

Which molecule is the immediate donor of the methyl group in the conversion of homocysteine to methionine?

  1. Betaine aldehyde
  2. N5-methyltetrahydrofolate Correct answer.
  3. Methylcobalamin
  4. S-adenosylmethionine

Explanation

Methionine synthase transfers the methyl group from N5-methyltetrahydrofolate to homocysteine, producing methionine and tetrahydrofolate. S-adenosylmethionine is the major methyl donor for other methylation reactions but is regenerated from methionine. Methylcobalamin is a cofactor, not the methyl donor.

Question 47Biochemistry

A competitive inhibitor is added to an enzyme-catalyzed reaction at a fixed substrate concentration below Km. Which kinetic change is expected?

  1. Increased apparent Km with unchanged Vmax Correct answer.
  2. Increased Vmax and increased Km
  3. Decreased Vmax with unchanged Km
  4. Decreased Km and decreased Vmax

Explanation

A competitive inhibitor binds the free enzyme and can be overcome by high substrate, so Vmax is unchanged while the apparent Km increases. At substrate concentrations below Km, the velocity falls because more enzyme is in the inhibited free form. Uncompetitive and noncompetitive inhibitors instead lower Vmax.

Question 48Biochemistry

During prolonged starvation, which substrate provides the majority of energy for the brain after several days of fasting?

  1. Free fatty acids bound to albumin
  2. Lactate produced by erythrocytes
  3. Ketone bodies Correct answer.
  4. Glucose derived from muscle glycogen

Explanation

After several days of fasting, hepatic ketogenesis supplies beta-hydroxybutyrate and acetoacetate, which the brain oxidizes and which reduce its glucose requirement. Muscle glycogen is depleted within about a day, and free fatty acids do not cross the blood-brain barrier efficiently.

Question 49Biochemistry

Which complex of the electron transport chain directly transfers electrons to molecular oxygen?

  1. Complex IV (cytochrome c oxidase) Correct answer.
  2. Complex III (cytochrome bc1)
  3. Complex I (NADH dehydrogenase)
  4. Complex II (succinate dehydrogenase)

Explanation

Cytochrome c oxidase accepts electrons from cytochrome c and reduces O2 to water at the final step of the chain. Complexes I, II and III transfer electrons between carriers but do not reduce oxygen directly.

Question 50Biochemistry

A child has recurrent infections and is found to have chronic granulomatous disease. This disorder most directly reflects defective production of which molecule?

  1. Hypochlorous acid generated by myeloperoxidase
  2. Hydrogen peroxide generated by catalase
  3. Superoxide generated by NADPH oxidase Correct answer.
  4. Nitric oxide generated by nitric oxide synthase

Explanation

Chronic granulomatous disease results from loss-of-function mutations in NADPH oxidase subunits, so phagocytes cannot generate the respiratory burst superoxide that is the precursor of microbicidal reactive oxygen species. Catalase and myeloperoxidase act downstream and are not the primary defect.