Practice 7 structured short- and long-answer medical questions on Chemical Pathology Sem 1. Review model answers for exam revision.
Answer: What is the most likely cause of his hyponatremia? (2 marks) - State two biochemical features characteristic of SIADH. (2 marks) - Explain why the urine osmolality is high despite hyponatremi - (2 marks) - Name the lung tumour most commonly associated with SIADH. (2 marks) - State one major risk of rapid correction of hyponatremi - (2 marks) --- ### ANSWER 1: - Most likely cause: (2 marks) Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH) secondary to ectopic ADH production by lung carcinoma (paraneoplastic syndrome). - Two biochemical features of SIADH: (2 marks) - Hyponatremia with low serum osmolality (<280 mOsm/kg) but inappropriately concentrated urine (urine osmolality >100 mOsm/kg, usually >300 mOsm/kg) - Elevated urine sodium (>40 mmol/L) despite hyponatremia, indicating continued sodium excretion due to volume expansion - Explanation of high urine osmolality: (2 marks) In SIADH, there is excessive and inappropriate secretion of ADH despite low serum osmolality. ADH acts on the collecting ducts of the kidney to increase water reabsorption, leading to concentrated urin - Normally, low serum osmolality should suppress ADH, resulting in dilute urine, but in SIADH this feedback mechanism is overridden. - Lung tumour most commonly associated with SIADH: (2 marks) Small Cell Lung Carcinoma (SCL - - accounts for approximately 75-80% of cases of ectopic ADH production. SCLC is of neuroendocrine origin and can synthesize and secrete ADH autonomously. - Major risk of rapid correction: (2 marks) Osmotic Demyelination Syndrome (ODS) , formerly called Central Pontine Myelinolysis. Rapid correction (>10-12 mmol/L in 24 hours) causes acute osmotic stress on brain cells that have adapted to chronic hyponatremia, leading to irreversible brainstem damage with quadriplegia, pseudobulbar palsy, and altered consciousness. ---
Answer: What is the most likely biochemical abnormality? (2 marks) - Briefly explain how thiazide diuretics cause hypokalemi - (2 marks) - Explain how chronic laxative abuse produces metabolic alkalosis. (3 marks) - Mention one common ECG change seen in hypokalemi - (2 marks) - State the first-line management for severe hypokalemi - (1 mark) --- ### ANSWER 2: - Most likely biochemical abnormality: (2 marks) Hypokalemic metabolic alkalosis with hypochloremia - combined effect of thiazide diuretic and chronic laxative abuse leading to severe potassium and chloride depletion. - Mechanism of thiazide-induced hypokalemia: (2 marks) Thiazide diuretics inhibit the Na-Cl cotransporter in the distal convoluted tubule, causing: - Increased sodium delivery to the collecting duct - Enhanced sodium reabsorption via epithelial sodium channels (ENa - - Increased potassium secretion in exchange for sodium (via ROMK channels) - Net result: urinary potassium loss and hypokalemia - Mechanism of laxative-induced metabolic alkalosis: (3 marks) Chronic laxative abuse causes: - Gastrointestinal bicarbonate loss initially - laxatives cause diarrhea with loss of bicarbonate-rich fluid from the colon - Volume depletion - chronic diarrhea leads to extracellular fluid loss - Contraction alkalosis - volume depletion stimulates aldosterone secretion, which:Increases renal hydrogen ion excretion (via H+-ATPase in collecting duct) - Increases bicarbonate reabsorption in proximal tubule - Enhances potassium loss, worsening hypokalemia - Hypokalemia perpetuates alkalosis - low potassium causes intracellular shift of hydrogen ions and increased renal acid excretion - Common ECG changes in hypokalemia: (2 marks) U waves (prominent U waves after T waves, best seen in V2-V3) Other changes include: ST depression, T wave flattening/inversion, prolonged QT interval, increased P wave amplitude - First-line management: (1 mark) Intravenous potassium chloride replacement - KCl infusion (rate usually 10-20 mmol/hour via peripheral line, or faster via central line in severe cases), with continuous cardiac monitoring. ---
Answer: Identify the acid–base disturbance present. (2 marks) - Explain the biochemical mechanism producing this metabolic acidosis. (3 marks) - Why is the PaCO₂ low in this patient? (2 marks) - What is the most likely underlying diagnosis? (2 marks) - State two causes of markedly elevated lactate in acute abdomen. (1 mark) --- ### ANSWER 3: - Acid-base disturbance: (2 marks) Severe metabolic acidosis with appropriate respiratory compensation (partially compensated metabolic acidosis with high anion gap) - Biochemical mechanism: (3 marks) The severe metabolic acidosis results from: - Lactic acidosis - absent femoral pulses indicate vascular insufficiency (likely acute mesenteric ischemia or ruptured abdominal aortic aneurysm) - Anaerobic metabolism - tissue hypoperfusion forces cells to undergo anaerobic glycolysis, producing lactate instead of CO₂ and water - High anion gap - accumulation of lactate (unmeasured anion) increases the anion gap - Shock state - inadequate tissue oxygen delivery leads to cellular hypoxia and multi-organ dysfunction with acid accumulation - Explanation of low PaCO₂: (2 marks) Respiratory compensation (Kussmaul breathing) - the low pH stimulates peripheral and central chemoreceptors, triggering hyperventilation to blow off CO₂ and partially correct the pH. The expected compensatory PaCO₂ can be calculated using Winter's formula: Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 (±2) = 1.5 × 7 + 8 = 18.5 mmHg (±2). The measured PaCO₂ of 26.3 mmHg indicates appropriate compensation. - Most likely underlying diagnosis: (2 marks) Acute mesenteric ischemia (due to superior mesenteric artery occlusion/thrombosis) Evidence supporting this: - Absent femoral pulses (peripheral vascular diseas - - Severe abdominal pain out of proportion to examination - Rigid distended abdomen (bowel necrosis/perforation) - Severe lactic acidosis (bowel infarction) - Shock state Alternative diagnosis: Ruptured abdominal aortic aneurysm with bowel ischemia - Two causes of elevated lactate in acute abdomen: (1 mark) - Bowel ischemia/infarction - mesenteric vascular occlusion - Septic shock - bowel perforation with peritonitis/sepsis ---
Answer: What acid–base disorder is present? (2 marks) - Explain the mechanism of respiratory alkalosis in head injury. (3 marks) - Why is the bicarbonate level slightly reduced after 3 days? (2 marks) - State one effect of respiratory alkalosis on cerebral blood flow. (2 marks) - Which area of the brain regulates the respiratory drive? (1 mark) --- ### ANSWER 4: - Acid-base disorder: (2 marks) Acute respiratory alkalosis with partial metabolic compensation (or respiratory alkalosis with compensatory metabolic acidosis if >3 days) - Mechanism of respiratory alkalosis in head injury: (3 marks) Head injury causes respiratory alkalosis through: - Direct brainstem stimulation - cerebral contusions and increased intracranial pressure (ICP) directly stimulate the respiratory center in the medulla - Central neurogenic hyperventilation - damage to inhibitory pathways or direct stimulation of respiratory centers causes uncontrolled hyperventilation - Compensatory response to increased ICP - hyperventilation may be a physiological attempt to reduce ICP (low CO₂ causes cerebral vasoconstriction, reducing cerebral blood volume and ICP) - Loss of normal respiratory control - disruption of cortical and subcortical modulation of breathing patterns - Why bicarbonate is reduced after 3 days: (2 marks) Metabolic (renal) compensation for chronic respiratory alkalosis. The kidneys respond to sustained hypocapnia by: - Decreasing renal hydrogen ion excretion - Decreasing bicarbonate reabsorption in the proximal tubule - Increasing bicarbonate excretion in urine This process takes 2-3 days to become significant, lowering serum bicarbonate by approximately 4-5 mmol/L for every 10 mmHg decrease in PaCO₂. - Effect on cerebral blood flow: (2 marks) Cerebral vasoconstriction with reduced cerebral blood flow (CBF) Mechanism: Low PaCO₂ increases pH in the cerebrospinal fluid and brain extracellular fluid, causing arteriolar smooth muscle constriction. For every 1 mmHg decrease in PaCO₂, cerebral blood flow decreases by approximately 2-4%. This can be therapeutic in acute head injury (reduces ICP) but excessive hyperventilation can cause cerebral ischemi - - Brain area regulating respiratory drive: (1 mark) Medulla oblongata (specifically the dorsal and ventral respiratory groups/centers in the medullary reticular formation) --- ## SECTION B: STANDARD SHORT ANSWER QUESTIONS (50 Marks Total)
Answer: Define the following terms as used in quality control: (4 marks) i) Accuracyii) Precisioniii) Sensitivityiv) Specificity - Explain the difference between internal and external quality control. (3 marks) - List three pre-analytical factors that can affect biochemical test results. (3 marks) --- ### ANSWER 5: - Definitions: (4 marks) i) Accuracy – The closeness of a measured value to the true or accepted reference valu - It indicates how correct a measurement is and reflects systematic error or bias. ii) Precision – The reproducibility or consistency of repeated measurements of the same sampl - It shows how close multiple measurements are to each other and reflects random error. iii) Sensitivity – The ability of a test to correctly identify individuals with a disease (true positive rate).Formula: Sensitivity = True Positives / (True Positives + False Negatives) × 100 iv) Specificity – The ability of a test to correctly identify individuals without a disease (true negative rate).Formula: Specificity = True Negatives / (True Negatives + False Positives) × 100 - Difference between Internal and External Quality Control: (3 marks) Internal Quality Control (IQC): - Performed within the laboratory on a daily basis - Uses control materials (known concentrations) analyzed alongside patient samples - Monitors day-to-day precision and detects analytical errors - Ensures consistency and reliability of results over time - Uses Levy-Jennings charts and Westgard rules External Quality Control (EQC/EQAS): - Involves inter-laboratory comparison programs (proficiency testing) - Identical samples distributed to multiple laboratories - Assesses accuracy and compares laboratory performance with peers - Identifies systematic errors, biases, and method-related problems - Conducted periodically (monthly/quarterly) - Three Pre-analytical Factors: (3 marks) - Patient preparation – Fasting status, exercise, posture, stress, circadian rhythm, diet, medication, alcohol consumption - Specimen collection – Incorrect tube type/order of draw, hemolysis, improper timing, prolonged tourniquet application, contamination from IV lines - Specimen handling/storage – Improper temperature, delayed processing, exposure to light, inadequate mixing, evaporation, microbial contamination ---
Answer: Calculate the anion gap for a patient with the following results: (3 marks) - Sodium: 138 mmol/L - Chloride: 98 mmol/L - Bicarbonate: 12 mmol/L - Interpret the acid-base status of a patient with these blood gas results: (4 marks) - pH: 7.28 - PaCO₂: 30 mmHg - HCO₃⁻: 14 mmol/L - List three causes of high anion gap metabolic acidosis. (3 marks) [hidden] --- ### ANSWER 6: - Anion Gap Calculation: (3 marks) Formula: Anion Gap = Na⁺ – (Cl⁻ + HCO₃⁻) Anion Gap = 138 – (98 + 12)Anion Gap = 138 – 110 Anion Gap = 28 mmol/L Normal range: 8-12 mmol/L (or 10-14 mmol/L depending on metho - Interpretation: Markedly elevated anion gap, consistent with high anion gap metabolic acidosis - Acid-Base Interpretation: (4 marks) Primary disorder: Metabolic Acidosis - pH 7.28 (acidotic, normal: 7.35-7.45) - HCO₃⁻ 14 mmol/L (low, normal: 22-26 mmol/L) - PaCO₂ 30 mmHg (low, normal: 35-45 mmHg) Compensation: Appropriate respiratory compensation The low PaCO₂ indicates hyperventilation to eliminate CO₂. Expected compensatory PaCO₂ using Winter's formula:Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 (±2)= 1.5 × 14 + 8= 29 mmHg (±2) = 27-31 mmHg Measured PaCO₂ (30 mmHg) falls within expected range, confirming appropriate respiratory compensation. Conclusion: Partially compensated high anion gap metabolic acidosis with appropriate respiratory compensation - Three Causes of High Anion Gap Metabolic Acidosis (MUDPILES): (3 marks) - Diabetic Ketoacidosis (DK - – accumulation of ketoacids (β-hydroxybutyrate and acetoacetat - due to insulin deficiency - Lactic Acidosis – from tissue hypoxia (Type A: sepsis, shock, severe hypoxemi - or metabolic causes (Type B: metformin, malignancy) - Renal Failure (Uremi - – accumulation of sulfates, phosphates, and organic acids due to decreased GFR Other causes: Methanol, Ethylene glycol, Salicylates (aspirin), Propylene glycol, Isoniazid ---
Answer: Interpret these results and state the lipid abnormalities present. (4 marks) - Calculate the VLDL cholesterol level. (2 marks) - What is the cardiovascular risk category for this patient? (2 marks) - Outline two lifestyle modifications you would recommen - (2 marks) --- ### ANSWER 7: - Interpretation and Lipid Abnormalities: (4 marks) - Hypercholesterolemia – Total cholesterol 280 mg/dL (Desirable: <200 mg/dL; High: ≥240 mg/dL) - Elevated LDL-Cholesterol – LDL 190 mg/dL (Optimal: <100 mg/dL; Very high: ≥190 mg/dL) - major atherogenic lipoprotein - Low HDL-Cholesterol – HDL 35 mg/dL (Normal for males: ≥40 mg/dL; Low: <40 mg/dL) - loss of cardioprotective effect - Hypertriglyceridemia – Triglycerides 275 mg/dL (Normal: <150 mg/dL; High: 200-499 mg/dL) Overall diagnosis: Mixed dyslipidemia/Combined hyperlipidemia (Type IIb hyperlipoproteinemi - - both cholesterol and triglycerides elevated - VLDL Cholesterol Calculation: (2 marks) Formula: VLDL = Triglycerides / 5 (Friedewald equation, valid when TG <400 mg/dL) VLDL = 275 / 5 VLDL = 55 mg/dL (Normal VLDL: 5-40 mg/dL) - This is elevated Verification: Total Cholesterol = LDL + HDL + VLDL280 = 190 + 35 + 55 - Cardiovascular Risk Category: (2 marks) VERY HIGH RISK for atherosclerotic cardiovascular disease (ASCV - Risk factors present: - LDL ≥190 mg/dL (automatic high-risk category requiring statin therapy) - Low HDL <40 mg/dL (independent risk factor) - Elevated triglycerides >200 mg/dL - Multiple lipid abnormalities (atherogenic dyslipidemi - - Male gender and age 50 years 10-year ASCVD risk likely >20% - Two Lifestyle Modifications: (2 marks) - Dietary modifications: Adopt Mediterranean or DASH diet - Reduce saturated fat to <7% of total calories - Eliminate trans fats - Increase soluble fiber (10-25 g/day from oats, beans, vegetables) - Increase omega-3 fatty acids (fatty fish 2× weekly) - Limit dietary cholesterol <200 mg/day - Reduce simple carbohydrates (helps lower triglycerides) - Regular physical activity: At least 150 minutes/week of moderate-intensity aerobic exercise (or 75 min vigorous) - Weight reduction if overweight (target BMI 18.5-24.9 kg/m²) - Exercise reduces triglycerides, increases HDL, and improves insulin sensitivity Additional recommendations: Smoking cessation (if applicable), alcohol moderation, stress management ---