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OMSB Physician Exam study materials
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OMSB Physician Exam practice questions

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Question 1Adult Cardiology

A 58-year-old man with no prior cardiac history presents with 2 hours of substernal chest pressure. His ECG shows 2 mm ST-segment elevation in leads V1–V4. Which coronary artery is most likely occluded?

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

Explanation

Anterior precordial leads V1–V4 reflect the anteroseptal wall, which is supplied by the left anterior descending artery. ST elevation in this territory therefore points to an LAD occlusion, typically a proximal or mid segment. Right and circumflex occlusions produce inferior or lateral changes rather than isolated V1–V4 elevation.

Question 2Adult Cardiology

Which finding on physical examination is most characteristic of severe aortic stenosis?

  1. A harsh crescendo–decrescendo systolic murmur radiating to the carotids Correct answer.
  2. A mid-diastolic rumble with an opening snap
  3. A decrescendo diastolic murmur at the left sternal edge
  4. A pansystolic murmur at the apex radiating to the axilla

Explanation

Aortic stenosis generates turbulent ejection flow across a narrowed valve, producing a harsh crescendo–decrescendo systolic murmur that classically radiates to the carotid arteries. The murmur peaks later and becomes softer in severity as stroke volume falls. The other described murmurs correspond to mitral regurgitation, mitral stenosis and aortic regurgitation.

Question 3Adult Cardiology

A 70-year-old woman has palpitations. Her ECG shows an irregularly irregular rhythm with no discernible P waves and a ventricular rate of 130/min. She is haemodynamically stable. Which is the most appropriate immediate management?

  1. Subcutaneous low-molecular-weight heparin alone
  2. Intravenous amiodarone infusion
  3. Urgent synchronised DC cardioversion
  4. Rate control with an intravenous beta-blocker Correct answer.

Explanation

In a stable patient with rapid atrial fibrillation, the priority is control of the ventricular rate, usually with an intravenous beta-blocker or a non-dihydropyridine calcium channel blocker. Electrical cardioversion is reserved for haemodynamic instability or refractory symptoms. Anticoagulation is assessed separately and does not replace rate control in the acute setting.

Question 4Adult Cardiology

Which ECG criterion is part of the diagnostic criteria for left ventricular hypertrophy?

  1. Delta wave with a short PR interval
  2. QTc greater than 440 ms
  3. Sokolow–Lyon voltage: S in V1 plus R in V5/V6 greater than 35 mm Correct answer.
  4. PR interval greater than 200 ms

Explanation

The Sokolow–Lyon criterion for left ventricular hypertrophy is a sum of the S wave in V1 and the R wave in V5 or V6 exceeding 35 mm. Other accepted criteria include the Cornell voltage and Romhilt–Estes score, often supported by repolarisation abnormalities. The remaining options describe conduction and repolarisation abnormalities rather than hypertrophy.

Question 5Adult Cardiology

A 62-year-old man with an ejection fraction of 25% is on optimal therapy but remains symptomatic. Which medication has been shown to reduce mortality in heart failure with reduced ejection fraction?

  1. Furosemide
  2. Ivabradine
  3. Digoxin
  4. Sacubitril–valsartan Correct answer.

Explanation

Sacubitril–valsartan, a combined neprilysin and angiotensin receptor inhibitor, reduces cardiovascular death and heart failure hospitalisation in reduced ejection fraction and is a guideline-recommended foundation therapy. Ivabradine reduces hospitalisation in selected patients but has not shown a mortality benefit. Digoxin and loop diuretics improve symptoms without improving survival.

Question 6Adult Cardiology

Which feature most strongly supports a diagnosis of acute pericarditis rather than ST-elevation myocardial infarction?

  1. New left bundle branch block on the ECG
  2. Widespread concave ST elevation with PR depression Correct answer.
  3. Reciprocal ST depression in inferior leads
  4. Regional wall motion abnormality on echocardiography

Explanation

Acute pericarditis classically produces widespread concave ST elevation with PR-segment depression, without reciprocal ST depression in a single territory. Myocardial infarction typically shows convex ST elevation confined to a coronary territory with reciprocal changes. Regional wall motion abnormality and new left bundle branch block favour ischaemia.

Question 7Adult Cardiology

A 30-year-old man has a resting blood pressure of 168/104 mmHg in both arms on three occasions. He reports episodic headache, palpitations and sweating. Which initial test is most appropriate to screen for a secondary cause?

  1. Plasma free metanephrines Correct answer.
  2. Renal artery duplex ultrasound
  3. Plasma aldosterone-to-renin ratio
  4. Overnight dexamethasone suppression test

Explanation

Episodic headache, palpitations and sweating in a young hypertensive patient suggest phaeochromocytoma, for which plasma free metanephrines are the most sensitive initial screening test. Aldosterone-to-renin ratio screens for primary aldosteronism, which presents with hypokalaemia rather than paroxysms. Cortisol testing and renal artery imaging address other causes that do not fit this clinical picture.

Question 8Adult Cardiology

Which mechanism best explains syncope in a patient with severe aortic stenosis during exertion?

  1. Excessive vagal tone causing sinus arrest
  2. Failure of cardiac output to rise because of a fixed outflow obstruction Correct answer.
  3. Baroreceptor-mediated vasodepression from carotid sinus hypersensitivity
  4. Acute coronary thrombosis during exercise

Explanation

In severe aortic stenosis, the fixed obstruction prevents stroke volume from increasing during exertion, so peripheral vasodilation produces a fall in blood pressure and cerebral hypoperfusion. This exertional syncope is a marker of severe disease and warrants prompt valve assessment. Carotid sinus hypersensitivity and vagal sinus arrest are unrelated mechanisms.

Question 9Adult Cardiology

A 68-year-old woman with a history of hypertension presents with tearing interscapular pain. Blood pressure is 180/100 mmHg in the right arm and 140/85 mmHg in the left arm. Which diagnosis should be suspected first?

  1. Acute pulmonary embolism
  2. Aortic dissection Correct answer.
  3. Pericardial tamponade
  4. Unstable angina

Explanation

Tearing interscapular pain with an interarm blood pressure difference in a hypertensive older patient is the classic presentation of aortic dissection. Immediate imaging with CT angiography or transoesophageal echocardiography is required, alongside blood pressure and heart rate control. The other diagnoses do not typically produce a significant interarm pressure gradient.

Question 10Adult Cardiology

Which finding is expected on ECG in a patient with hyperkalaemia of 7.0 mmol/L?

  1. Peaked T waves with a widened QRS complex Correct answer.
  2. Diffuse ST elevation with PR depression
  3. Sawtooth flutter waves
  4. Prominent U waves

Explanation

Hyperkalaemia initially produces tall, peaked T waves and, as potassium rises further, QRS widening, PR prolongation and loss of P waves, with a risk of sine-wave pattern and cardiac arrest. Diffuse ST elevation with PR depression suggests pericarditis, sawtooth waves indicate atrial flutter, and U waves suggest hypokalaemia.

Question 11Adult Cardiology

A 55-year-old man with heart failure and an ejection fraction of 30% has a QRS duration of 160 ms with left bundle branch morphology and remains symptomatic despite optimal medical therapy. Which intervention is most likely to improve symptoms and survival?

  1. Cardiac resynchronisation therapy Correct answer.
  2. Radiofrequency ablation of the AV node
  3. Implantable cardioverter-defibrillator alone
  4. Amiodarone therapy

Explanation

Cardiac resynchronisation therapy is indicated for symptomatic heart failure with reduced ejection fraction and a broad QRS, particularly with left bundle branch block, and improves symptoms and survival. An ICD addresses sudden death risk but does not resynchronise the ventricles. AV node ablation and amiodarone do not provide the same benefit.

Question 12Adult Cardiology

Which physical sign is most suggestive of cardiac tamponade?

  1. Wide pulse pressure with a bounding pulse
  2. Pulsus paradoxus with elevated jugular venous pressure Correct answer.
  3. Slow-rising carotid pulse with a narrow pulse pressure
  4. Continuous machinery murmur

Explanation

Cardiac tamponade raises intrapericardial pressure, impairing diastolic filling and causing elevated jugular venous pressure with pulsus paradoxus. Beck's triad also includes hypotension and muffled heart sounds. A bounding pulse suggests aortic regurgitation, a slow-rising pulse suggests aortic stenosis, and a machinery murmur suggests patent ductus arteriosus.

Question 13Adult Cardiology

A 45-year-old woman has a systolic murmur and a fixed, widely split second heart sound. Which lesion is most likely?

  1. Ventricular septal defect
  2. Tricuspid regurgitation
  3. Atrial septal defect Correct answer.
  4. Mitral valve prolapse

Explanation

A fixed, widely split second heart sound occurs with an atrial septal defect because the left-to-right shunt equalises the respiratory variation in right ventricular filling. The associated murmur is a pulmonary flow murmur. Mitral valve prolapse causes a mid-systolic click with late systolic murmur, and ventricular septal defect causes a harsh pansystolic murmur.

Question 14Adult Cardiology

Which of the following is a major risk factor for infective endocarditis in a patient with a prosthetic valve?

  1. Recent dental extraction without prophylaxis Correct answer.
  2. Regular aerobic exercise
  3. Mild iron deficiency anaemia
  4. Well-controlled hypertension

Explanation

Transient bacteraemia after dental procedures can seed a prosthetic valve, making recent dental work without appropriate prophylaxis a major risk factor for infective endocarditis. Prosthetic valves carry the highest risk, and guidelines recommend prophylaxis before high-risk dental procedures. Hypertension, exercise and mild anaemia do not predispose to endocarditis.

Question 15Adult Cardiology

A 60-year-old man with chest pain has an ECG showing ST elevation in leads II, III and aVF. Which complication should be monitored for because of the likely involved artery?

  1. Left bundle branch block
  2. First-degree heart block
  3. Atrial flutter
  4. Complete heart block Correct answer.

Explanation

Inferior ST elevation usually reflects right coronary artery occlusion, which supplies the atrioventricular node in most people. Complete heart block is therefore a recognised complication and requires monitoring and possible pacing. Left bundle branch block relates more to anterior infarction, and atrial flutter is not a specific complication of inferior infarction.

Question 16Adult Cardiology

Which statement about statin therapy in primary prevention is correct?

  1. Statins are contraindicated in patients with diabetes
  2. Statins have no role in patients older than 75 years
  3. Statins reduce cardiovascular events mainly by lowering LDL cholesterol Correct answer.
  4. Statins should be started only after a myocardial infarction

Explanation

Statins lower LDL cholesterol by inhibiting HMG-CoA reductase, and this reduction is the principal mechanism by which they decrease cardiovascular events in both primary and secondary prevention. Their use is guided by overall cardiovascular risk, and patients with diabetes often benefit substantially. Age alone does not contraindicate therapy.

Question 17Adult Cardiology

A 72-year-old man with known coronary disease presents with sudden collapse. The monitor shows a wide-complex tachycardia at 220/min with no palpable pulse. Which is the immediate priority?

  1. Intravenous adenosine 6 mg
  2. Synchronised cardioversion at 100 J
  3. Defibrillation with an unsynchronised shock Correct answer.
  4. Carotid sinus massage

Explanation

A wide-complex tachycardia without a pulse is treated as ventricular fibrillation or pulseless ventricular tachycardia, requiring immediate unsynchronised defibrillation and high-quality CPR. Synchronised cardioversion applies only to a patient with a pulse and a shockable organised rhythm. Adenosine and vagal manoeuvres are inappropriate in pulseless wide-complex rhythms.

Question 18Anesthesia

During preoxygenation before rapid sequence induction in a healthy adult, which endpoint best confirms that functional residual capacity has been adequately denitrogenated?

  1. Respiratory rate below 12 breaths per minute
  2. End-tidal oxygen fraction above 0.90 Correct answer.
  3. End-tidal carbon dioxide below 30 mmHg
  4. Oxygen saturation of 100% on pulse oximetry

Explanation

Denitrogenation is confirmed by measuring the end-tidal oxygen fraction, which rises toward 0.90 or higher as nitrogen in the functional residual capacity is replaced by oxygen. Pulse oximetry cannot detect residual nitrogen because hemoglobin is already fully saturated in a healthy patient breathing room air.

Question 19Anesthesia

A 70 kg adult receives a peripheral nerve block with 30 mL of 0.5% bupivacaine. What is the total dose of local anesthetic administered?

  1. 450 mg
  2. 150 mg Correct answer.
  3. 300 mg
  4. 75 mg

Explanation

A 0.5% solution contains 5 mg per mL, so 30 mL delivers 150 mg. Calculating the milligram dose is essential before comparing it with the maximum safe dose for the patient's weight.

Question 20Anesthesia

Which volatile agent is most appropriate for inhalational induction of anesthesia in a child because of its low blood-gas solubility and non-pungent odor?

  1. Nitrous oxide
  2. Isoflurane
  3. Sevoflurane Correct answer.
  4. Desflurane

Explanation

Sevoflurane has a low blood-gas solubility, giving rapid uptake and induction, and it is non-irritant to the airway, which makes it suitable for mask induction in children. Desflurane and isoflurane are pungent and can provoke coughing, laryngospasm, and breath-holding.

Question 21Anesthesia

Which finding on a preoperative airway assessment most strongly predicts difficult mask ventilation in an adult?

  1. Body mass index of 32 kg/m2
  2. Mallampati class II
  3. Presence of a short, thick neck with a beard Correct answer.
  4. Thyromental distance of 7 cm

Explanation

A short, thick neck combined with a beard is a recognized independent predictor of difficult mask ventilation because beard hair prevents a seal and the neck geometry obstructs the airway. The other listed findings are either normal or weak predictors.

Question 22Anesthesia

Which monitoring modality is the standard for detecting intraoperative malignant hyperthermia at its earliest stage?

  1. Transesophageal echocardiography
  2. Peripheral nerve stimulation
  3. Continuous end-tidal capnography Correct answer.
  4. Bispectral index monitoring

Explanation

The earliest sign of malignant hyperthermia is an unexpected rise in end-tidal carbon dioxide despite adequate ventilation, reflecting a hypermetabolic state. Temperature rise is a late finding, so capnography provides the earliest reliable detection.

Question 23Anesthesia

An adult with a history of gastroesophageal reflux requires emergency surgery with a full stomach. Which induction strategy minimizes the risk of pulmonary aspiration?

  1. Awake fiberoptic intubation under topical anesthesia with sedation
  2. Rapid sequence induction with preoxygenation and endotracheal intubation Correct answer.
  3. Rapid sequence induction with cricoid pressure and a supraglottic airway
  4. Inhalational induction with spontaneous ventilation and a laryngeal mask

Explanation

Rapid sequence induction with preoxygenation, avoidance of positive-pressure ventilation before intubation, and a cuffed endotracheal tube is the standard approach to protect the airway in a patient at risk of aspiration. A supraglottic airway does not reliably seal the trachea against regurgitated gastric contents.

Question 24Anesthesia

A patient receiving a spinal anesthetic develops hypotension with bradycardia shortly after the block is placed. Which mechanism best explains this presentation?

  1. Blockade of cardiac sympathetic fibers causing unopposed vagal tone Correct answer.
  2. Acute hypovolemia from hemorrhage
  3. Direct myocardial depression from the local anesthetic
  4. Anaphylaxis to the local anesthetic

Explanation

High neuraxial blockade interrupts sympathetic fibers to the heart, reducing venous return and leaving vagal tone unopposed, which produces hypotension with bradycardia. Treatment includes fluids, vasopressors, and anticholinergics when bradycardia is prominent.

Question 25Anesthesia

Which nerve block is most appropriate for anesthesia of the anterior two-thirds of the tongue and the lingual gingiva during a dental procedure?

  1. Greater palatine nerve block
  2. Inferior alveolar nerve block
  3. Buccal nerve block
  4. Lingual nerve block Correct answer.

Explanation

The lingual nerve supplies general sensation to the anterior two-thirds of the tongue and the lingual gingiva. An inferior alveolar block anesthetizes the teeth and chin but spares the tongue unless the lingual nerve is also blocked.

Question 26Anesthesia

Which finding on capnography is most characteristic of an esophageal intubation?

  1. Absent or rapidly fading end-tidal carbon dioxide with each ventilation Correct answer.
  2. An elevated end-tidal carbon dioxide with a prolonged expiratory phase
  3. A waveform with a gradual upsloping plateau
  4. A sudden fall in end-tidal carbon dioxide to zero after a stable period

Explanation

Esophageal placement produces no sustained carbon dioxide because the esophagus has no gas exchange, so the trace is flat or shows only brief low readings from gas in the stomach. Sustained capnography over several breaths confirms tracheal placement.

Question 27Anesthesia

A patient with myasthenia gravis is scheduled for thymectomy. Which neuromuscular management principle is most appropriate?

  1. Use a reduced dose of non-depolarizing blocker and monitor with a peripheral nerve stimulator Correct answer.
  2. Administer neostigmine routinely before reversal is needed
  3. Avoid all neuromuscular blockers and rely on deep inhalational anesthesia
  4. Use a standard dose of succinylcholine for induction

Explanation

Patients with myasthenia gravis have reduced acetylcholine receptors and show increased sensitivity to non-depolarizing neuromuscular blockers, so doses should be reduced and titrated with quantitative neuromuscular monitoring. Succinylcholine response is variable and can be resistant or exaggerated.

Question 28Anesthesia

Which agent is the first-line treatment for an acute malignant hyperthermia crisis after dantrolene is prepared?

  1. Propranolol
  2. Amiodarone
  3. Sodium bicarbonate
  4. Dantrolene sodium Correct answer.

Explanation

Dantrolene is the specific antidote for malignant hyperthermia because it inhibits calcium release from the sarcoplasmic reticulum. Supportive measures include stopping triggering agents, hyperventilation with 100% oxygen, cooling, and treating hyperkalemia and arrhythmias.

Question 29Anesthesia

A patient with severe aortic stenosis is undergoing minor surgery under general anesthesia. Which hemodynamic goal is most important to maintain?

  1. Deep vasodilation with a mean arterial pressure of 55 mmHg
  2. Tachycardia and reduced afterload
  3. Normal to high systemic vascular resistance with maintenance of sinus rhythm Correct answer.
  4. Low filling pressures and mild hypotension

Explanation

In aortic stenosis, the fixed outflow obstruction makes cardiac output dependent on preload, atrial contribution, and adequate afterload. Maintaining sinus rhythm and systemic vascular resistance preserves coronary perfusion and avoids ischemia.

Question 30Anesthesia

A patient with a suspected difficult airway is breathing spontaneously and has a predicted inability to ventilate by mask. Which plan is safest?

  1. Induce general anesthesia and attempt direct laryngoscopy twice before waking the patient
  2. Perform awake fiberoptic intubation with topical anesthesia while preserving spontaneous ventilation Correct answer.
  3. Administer a depolarizing blocker and attempt blind nasal intubation
  4. Proceed with a supraglottic airway as the primary definitive airway

Explanation

When both intubation and mask ventilation are predicted to be difficult, the airway should be secured while the patient maintains spontaneous ventilation, typically with awake fiberoptic intubation. This avoids creating a cannot-intubate, cannot-ventilate situation.

Question 31Anesthesia

A morbidly obese patient is being ventilated under general anesthesia. Which ventilator strategy best reduces atelectasis and improves oxygenation?

  1. Tidal volume based on ideal body weight with recruitment and appropriate positive end-expiratory pressure Correct answer.
  2. Pressure support ventilation with the patient breathing spontaneously through an endotracheal tube
  3. Low tidal volume with zero positive end-expiratory pressure
  4. High tidal volume based on actual body weight without positive end-expiratory pressure

Explanation

Ventilation should be based on ideal body weight to avoid volutrauma, and recruitment maneuvers with positive end-expiratory pressure help counteract the atelectasis that is common in obesity. This improves functional residual capacity and oxygenation.

Question 32Anesthesia

During a laparoscopic procedure, end-tidal carbon dioxide rises and the patient becomes tachycardic and hypotensive. Which complication should be suspected first?

  1. Malignant hyperthermia
  2. Anaphylaxis to latex
  3. Tension pneumothorax
  4. Carbon dioxide embolism Correct answer.

Explanation

Carbon dioxide embolism during laparoscopy can present with a sudden rise in end-tidal carbon dioxide followed by a rapid fall, tachycardia, hypotension, and desaturation. Immediate management includes stopping insufflation, administering 100% oxygen, and supporting the circulation.

Question 33Anesthesia

A patient with chronic obstructive pulmonary disease is emerging from general anesthesia. Which strategy best reduces the risk of postoperative respiratory failure?

  1. Delay extubation until the patient is fully awake regardless of ventilation
  2. Use opioid-sparing analgesia and titrate reversal of neuromuscular blockade under quantitative monitoring Correct answer.
  3. Administer high-dose opioids to suppress cough
  4. Extubate deeply without assessing respiratory effort

Explanation

Opioid-sparing analgesia and complete reversal of neuromuscular blockade guided by quantitative monitoring reduce respiratory depression and residual weakness in patients with COPD. Adequate analgesia allows effective coughing and clearance of secretions.

Question 34Anesthesia

Which statement best describes the mechanism by which neostigmine reverses non-depolarizing neuromuscular blockade?

  1. It inhibits acetylcholinesterase, increasing acetylcholine at the neuromuscular junction Correct answer.
  2. It enhances hepatic metabolism of the non-depolarizing agent
  3. It directly displaces the blocker from the nicotinic receptor
  4. It blocks presynaptic calcium channels to reduce acetylcholine release

Explanation

Neostigmine inhibits acetylcholinesterase, so acetylcholine accumulates at the neuromuscular junction and competes more effectively with the non-depolarizing blocker. An antimuscarinic agent is usually co-administered to prevent bradycardia and secretions.

Question 35Biochemistry

A newborn presents with severe fasting hypoglycemia, hepatomegaly, and lactic acidosis. Enzyme studies show deficient glucose-6-phosphatase activity. Which substrate accumulates most directly because this enzyme is missing?

  1. Pyruvate
  2. Acetyl-CoA
  3. Fructose-1,6-bisphosphate
  4. Glucose-6-phosphate Correct answer.

Explanation

Glucose-6-phosphatase catalyzes the final step of both glycogenolysis and gluconeogenesis, converting glucose-6-phosphate to free glucose in the liver. When it is deficient (von Gierke disease), glucose-6-phosphate accumulates and is shunted into glycolysis and glycogen, producing lactic acidosis and hepatomegaly. The other listed metabolites lie upstream or in separate pathways and are not the immediate trapped substrate.

Question 36Biochemistry

During a prolonged fast, a patient's blood glucose is maintained largely by hepatic gluconeogenesis. Which enzyme catalyzes the rate-limiting committed step of gluconeogenesis from pyruvate?

  1. Phosphofructokinase-1
  2. Pyruvate carboxylase Correct answer.
  3. Fructose-1,6-bisphosphatase
  4. Pyruvate kinase

Explanation

Pyruvate carboxylase converts pyruvate to oxaloacetate in mitochondria and is the first committed, regulated step unique to gluconeogenesis; it requires biotin and acetyl-CoA as an activator. The subsequent decarboxylation and dephosphorylation steps complete the bypass of pyruvate kinase. The other options are glycolytic enzymes that would oppose gluconeogenesis.

Question 37Biochemistry

A patient with a defect in the carnitine shuttle presents with muscle weakness and hypoketotic hypoglycemia during fasting. What is the direct consequence of impaired carnitine palmitoyltransferase I activity in hepatocytes?

  1. Increased mitochondrial beta-oxidation of long-chain fatty acids
  2. Accelerated ketone body synthesis from acetyl-CoA
  3. Failure of long-chain fatty acyl groups to enter mitochondria for oxidation Correct answer.
  4. Enhanced conversion of fatty acids to glucose by gluconeogenesis

Explanation

Carnitine palmitoyltransferase I forms acylcarnitine on the outer mitochondrial membrane, which is required for long-chain fatty acids to cross into the matrix for beta-oxidation. Blocking this step reduces acetyl-CoA and ketone production, so fasting hypoglycemia is hypoketotic and fatty acids cannot be used for energy. Fatty acids cannot be converted to glucose because acetyl-CoA cannot be used for net gluconeogenesis in humans.

Question 38Biochemistry

A 30-year-old man develops hemolytic anemia after taking an antimalarial drug. His erythrocytes lack glucose-6-phosphate dehydrogenase. Which metabolic consequence best explains the hemolysis?

  1. Impaired regeneration of reduced glutathione, leaving cells vulnerable to oxidative damage Correct answer.
  2. Accumulation of glycogen within erythrocytes
  3. Deficient synthesis of heme in the bone marrow
  4. Failure of the erythrocyte to generate ATP by glycolysis

Explanation

G6PD produces NADPH in the pentose phosphate pathway, and NADPH is required by glutathione reductase to regenerate reduced glutathione that detoxifies reactive oxygen species. Without adequate NADPH, oxidative stress damages hemoglobin and the membrane, causing hemolysis after exposure to oxidizing drugs or fava beans. Glycolysis and ATP production remain intact in this disorder.

Question 39Biochemistry

A researcher studies the effect of an uncompetitive inhibitor on a purified enzyme. Which kinetic pattern is expected when substrate concentration is varied?

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

Explanation

An uncompetitive inhibitor binds only the enzyme-substrate complex, effectively lowering both apparent Km and apparent Vmax by the same factor, so parallel lines appear on a Lineweaver-Burk plot. This pattern distinguishes it from competitive inhibition, which raises Km with unchanged Vmax. The other options describe competitive, noncompetitive, or no inhibition.

Question 40Biochemistry

A child with phenylketonuria is placed on a phenylalanine-restricted diet. Deficiency of phenylalanine hydroxylase limits synthesis of which amino acid, making it conditionally essential in this patient?

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

Explanation

Phenylalanine hydroxylase converts phenylalanine to tyrosine using tetrahydrobiopterin as a cofactor. When this enzyme is deficient, tyrosine cannot be made and must be supplied in the diet, making it conditionally essential. The other amino acids are synthesized by unrelated pathways and are not the product of this reaction.

Question 41Biochemistry

A patient's laboratory results show elevated homocysteine and methylmalonic acid with normal serum folate. Which vitamin deficiency most likely explains this combined pattern?

  1. Vitamin B1 (thiamine)
  2. Vitamin B6 (pyridoxine)
  3. Vitamin C (ascorbate)
  4. Vitamin B12 (cobalamin) Correct answer.

Explanation

Vitamin B12 is a cofactor for methionine synthase, which remethylates homocysteine to methionine, and for methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA. Deficiency therefore raises both homocysteine and methylmalonic acid. Folate deficiency raises homocysteine alone because methylmalonyl-CoA mutase does not require folate.

Question 42Biochemistry

A 2-year-old presents with recurrent fractures, blue sclerae, and hearing loss. Collagen analysis reveals a defect in type I collagen. Which post-translational modification is most directly impaired in this disorder?

  1. Phosphorylation of serine residues
  2. Gamma-carboxylation of glutamate residues
  3. Hydroxylation of proline and lysine residues Correct answer.
  4. Acetylation of lysine residues on histones

Explanation

Collagen triple-helix stability requires hydroxylation of proline and lysine residues, reactions performed by prolyl and lysyl hydroxylases that depend on vitamin C, iron, and alpha-ketoglutarate. Defective hydroxylation or cross-linking weakens type I collagen, producing osteogenesis imperfecta features such as fractures and blue sclerae. The other modifications belong to different protein systems.

Question 43Biochemistry

A patient with acute pancreatitis has a serum sample with markedly elevated triglycerides. Which lipoprotein class is most characteristically increased in familial hypertriglyceridemia and carries the majority of circulating triglyceride in the fasting state?

  1. Low-density lipoprotein
  2. Chylomicron remnants
  3. High-density lipoprotein
  4. Very-low-density lipoprotein Correct answer.

Explanation

In the fasting state, very-low-density lipoprotein is the principal triglyceride carrier, transporting endogenously synthesized triglyceride from the liver to peripheral tissues. Chylomicrons carry dietary triglyceride but are normally cleared within hours after a meal. Familial hypertriglyceridemia reflects overproduction or impaired clearance of VLDL.

Question 44Biochemistry

A newborn screening test shows elevated immunoreactive trypsinogen, and sweat chloride testing later confirms cystic fibrosis. A defect in CFTR most directly impairs which process in epithelial cells?

  1. Chloride transport across the apical membrane Correct answer.
  2. Sodium-potassium ATPase activity on the basolateral membrane
  3. Urea transport in the collecting duct
  4. Glucose reabsorption in the proximal tubule

Explanation

CFTR is an apical membrane chloride channel that, when defective, reduces chloride secretion and secondarily increases sodium and water reabsorption, producing thick secretions. This explains elevated sweat chloride and inspissated mucus in lungs and pancreas. The other processes involve distinct transporters not encoded by CFTR.

Question 45Biochemistry

During intense exercise, skeletal muscle generates lactate that is later used by the liver to produce glucose. Which set of reactions correctly describes the Cori cycle?

  1. Muscle converts glucose to alanine, which the kidney excretes unchanged
  2. Muscle converts lactate to glucose, which the liver oxidizes
  3. Liver converts lactate to ketone bodies, which muscle oxidizes
  4. Liver converts lactate to glucose, which muscle uses Correct answer.

Explanation

In the Cori cycle, anaerobic glycolysis in muscle produces lactate, which travels to the liver and is converted back to glucose by gluconeogenesis. The glucose returns to muscle for reuse, shifting the metabolic cost to the liver. The alanine cycle is a separate interorgan pathway involving amino group transfer.

Question 46Biochemistry

A patient with ornithine transcarbamylase deficiency presents with hyperammonemia. Which biochemical consequence directly explains the neurologic toxicity of ammonia?

  1. Ammonia stimulates glycolysis by activating pyruvate kinase
  2. Ammonia directly inhibits hemoglobin synthesis in erythrocytes
  3. Ammonia increases mitochondrial calcium storage
  4. Ammonia depletes alpha-ketoglutarate, impairing the tricarboxylic acid cycle and ATP production Correct answer.

Explanation

Ammonia is detoxified by glutamate dehydrogenase and glutamine synthetase, reactions that consume alpha-ketoglutarate and glutamate. Depletion of alpha-ketoglutarate slows the TCA cycle and oxidative phosphorylation, contributing to cerebral energy failure and edema. The other options do not describe established mechanisms of ammonia neurotoxicity.

Question 47Biochemistry

A patient with severe malnutrition has low serum albumin and edema. Which change in Starling forces best explains the edema?

  1. Increased interstitial oncotic pressure from lymphatic obstruction
  2. Decreased plasma oncotic pressure Correct answer.
  3. Decreased capillary permeability to water
  4. Increased capillary hydrostatic pressure

Explanation

Albumin is the major contributor to plasma colloid oncotic pressure, which draws fluid back into capillaries. Hypoalbuminemia lowers this oncotic pressure, favoring net filtration into the interstitium and producing edema. Hydrostatic pressure and lymphatic obstruction cause edema by different mechanisms.

Question 48Biochemistry

A 45-year-old with a history of recurrent calcium oxalate kidney stones is found to have elevated urinary oxalate. Which enzyme defect in the liver would most directly increase oxalate production?

  1. Deficiency of alanine aminotransferase
  2. Deficiency of glutamate dehydrogenase
  3. Deficiency of alanine-glyoxylate aminotransferase Correct answer.
  4. Deficiency of lactate dehydrogenase

Explanation

Alanine-glyoxylate aminotransferase normally transaminates glyoxylate to glycine in peroxisomes. When it is deficient (primary hyperoxaluria type 1), glyoxylate accumulates and is oxidized to oxalate, which precipitates with calcium in the kidney. The other enzymes do not participate in glyoxylate detoxification.

Question 49Biochemistry

During a marathon, a runner's muscle relies on anaerobic glycolysis. Which allosteric change most directly activates phosphofructokinase-1 in exercising muscle?

  1. Falling pH and rising ATP
  2. Falling ATP and rising AMP Correct answer.
  3. Rising ATP and falling AMP
  4. Rising citrate and falling fructose-2,6-bisphosphate

Explanation

Phosphofructokinase-1 is inhibited by high ATP and citrate and activated by AMP and fructose-2,6-bisphosphate. During exercise, ATP falls and AMP rises, signaling low energy charge and stimulating glycolytic flux. Acidosis and high ATP would instead inhibit the enzyme.

Question 50Biochemistry

A patient with a mitochondrial DNA mutation affecting complex I of the electron transport chain presents with lactic acidosis and muscle fatigue. Which statement best explains the lactic acidosis?

  1. Lactate is produced because the TCA cycle is overactivated
  2. Excess oxygen is converted directly to lactate in mitochondria
  3. Impaired oxidative phosphorylation shifts pyruvate toward lactate production Correct answer.
  4. Increased complex I activity enhances pyruvate oxidation

Explanation

When electron transport is impaired, NADH cannot be reoxidized efficiently, so the NADH/NAD+ ratio rises and pyruvate is reduced to lactate by lactate dehydrogenase to regenerate NAD+. This regenerates the NAD+ needed for glycolysis and produces lactic acidosis. Oxygen is not converted directly to lactate.