10 clinical MCQs in Chemical Pathology. : SIADH / Hyponatremia (10 marks) An elderly male smoker is admitted with acute confusion.
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Q1. : SIADH / Hyponatremia (10 marks) An elderly male smoker is admitted with acute confusion. Examination reveals digital clubbing and a right-sided pleural effusion. Laboratory results show: - Serum sodium: 118 mmol/L - Serum osmolality: 236 mOsm/kg - Urine osmolality: 350 mOsm/kg - Urine sodium: 50 mmol/L - Urea and creatinine: normal Chest radiograph shows a right lower-zone mass suggestive of carcinoma.
- 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)
Answer: What is the most likely cause of his hyponatremia? (2 marks)
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Q2. : Thiazide + Laxative Abuse (Hypokalemic Metabolic Alkalosis) (10 marks) A 67-year-old woman presents with profound muscle weakness. She has a long history of laxative use and was recently started on a thiazide diuretic. Investigations show severe hypokalemia and metabolic alkalosis.
- 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
Answer: What is the most likely biochemical abnormality? (2 marks)
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Q3. : Shock + Acute Abdomen (Severe Metabolic Acidosis) (10 marks) A 60-year-old man presents with severe abdominal pain for 24 hours. He is shocked, has a rigid distended abdomen, and absent femoral pulses. Arterial blood gases show: - pH: 7.05 - PaCO₂: 26.3 mmHg - PaO₂: 90 mmHg - HCO₃⁻: 7 mmol/L
- 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:
Answer: Identify the acid–base disturbance present. (2 marks)
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Q4. : Head Injury (Respiratory Alkalosis) (10 marks) A young woman is admitted unconscious following head trauma. CT scan reveals extensive cerebral contusions. She is hyperventilating at 38 breaths per minute. Arterial blood gas shows: - PaCO₂: 29 mmHg - HCO₃⁻: 19 mmol/L
- 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:
Answer: What acid–base disorder is present? (2 marks)
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Q5. : Quality Control in Clinical Chemistry (10 marks)
- 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
Answer: Define the following terms as used in quality control: (4 marks) i) Accuracyii) Precisioniii) Sensitivityiv) Specificity
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Q6. : Acid-Base Balance (10 marks)
- 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:
- Interpretation: Markedly elevated anion gap, consistent with high anion gap metabolic acidosis
- Three Causes of High Anion Gap Metabolic Acidosis (MUDPILES): (3 marks) - Diabetic Ketoacidosis (DK
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
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Q7. : Lipid Profile Interpretation (10 marks) A 50-year-old male patient presents with the following lipid profile results: - Total Cholesterol: 280 mg/dL - LDL-Cholesterol: 190 mg/dL - HDL-Cholesterol: 35 mg/dL - Triglycerides: 275 mg/dL
- 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:
Answer: Interpret these results and state the lipid abnormalities present. (4 marks)
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Q8. : Liver Function Tests (10 marks)
- Differentiate between hepatocellular and cholestatic patterns of liver enzyme elevation. (4 marks)
- What is the significance of an AST/ALT ratio >2:1? (2 marks) --- ### ANSWER 8:
- Classification and Causes: (4 marks) Type of Jaundice: HEPATOCELLULAR (HEPATI
- Significance of AST/ALT Ratio >2:1: (2 marks) Highly suggestive of ALCOHOLIC LIVER DISEASE
Answer: Differentiate between hepatocellular and cholestatic patterns of liver enzyme elevation. (4 marks)
Explanation: AST/ALT ratio 2:1 has approximately 70% specificity for alcoholic liver disease - AST/ALT ratio 3:1 is nearly pathognomonic for alcoholic hepatitis - Mechanism: Chronic alcohol consumption depletes pyridoxal-5-phosphate (vitamin B6), a cofactor required for ALT synthesis - This leads to disproportionate reduction in ALT activity - Alcohol also causes mitochondrial damage, releasing mitochondrial AST (mAST) into circulation - AST is also less specific to liver (found in heart, muscle, kidney, brain), so may be elevated from other alcohol-related organ damage - However, both AST and ALT are usually <300 U/L in alcoholic hepatitis (unlike viral hepatitis where levels can exceed 1000 U/L) ---
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Q9. : Cardiac Biomarkers (10 marks)
- Explain the pattern of cardiac biomarker elevation following acute myocardial infarction for: (6 marks) i) Troponin (cTnI or cTnT)ii) CK-MBiii) Myoglobin
- What is the clinical significance of elevated BNP (B-type Natriuretic Peptide)? (4 marks) --- ### ANSWER 9:
- Clinical Significance of Elevated BNP: (4 marks) BNP (B-type Natriuretic Peptid
- is released from cardiac ventricular myocytes in response to ventricular wall stretch and pressure overloa
- Obesity decreases BNP levels. --- ## SECTION C: CLINICAL BIOCHEMISTRY INVESTIGATIONS (10 Marks)
Answer: Explain the pattern of cardiac biomarker elevation following acute myocardial infarction for: (6 marks) i) Troponin (cTnI or cTnT)ii) CK-MBiii) Myoglobin
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Q10. : Diagnostic Test Panels (10 marks) For each of the following clinical conditions, list the key biochemical investigations you would request
- Suspected glomerulonephritis (3 marks)
- Suspected rhabdomyolysis (2 marks)
- Suspected hemolytic anemia (2 marks)
- Suspected multiple myeloma (3 marks) --- ### ANSWER 10:
- - Autoantibodies: ANA (Antinuclear antibody) - for systemic lupus erythematosus - Anti-dsDNA (anti-double stranded DN
Answer: Suspected glomerulonephritis (3 marks)