Master Physiology
for FMGE

What the FMGE Tests in Physiology

The FMGE Physiology exam tests the application of normal and abnormal physiological principles to clinical scenarios. Candidates must interpret arterial blood gas values, recognise electrolyte disturbances (e.g., hyperkalaemia >5.5 mmol/L), and understand compensatory mechanisms in acid-base disorders (e.g., Winter's formula for metabolic acidosis). Questions often present a patient with a condition (e.g., Cushing's syndrome, Addisonian crisis, diabetic ketoacidosis) and ask for the underlying physiological derangement, expected lab values, or first-line management (e.g., IV insulin and fluid resuscitation for DKA). Emphasis is on integrated systems: renal, respiratory, cardiovascular, and endocrine physiology, with direct links to clinical decision-making such as when to administer IV calcium gluconate for hyperkalaemia or identify third-degree heart block on an ECG.

High-Yield Concepts

• Acid-Base Disorders: Winter's Formula: In metabolic acidosis, the expected respiratory compensation is calculated by Winter's formula: PaCO₂ = (1.5 × [HCO₃⁻]) + 8 ± 2. If measured PaCO₂ is higher, a concurrent respiratory acidosis exists; if lower, respiratory alkalosis. For example, in DKA with HCO₃⁻ 10 mmol/L, expected PaCO₂ is 23 ± 2 mmHg.
• Hyperkalaemia: ECG Changes and Emergency Management: ECG changes include peaked T waves, loss of P wave, wide QRS, and sine wave pattern. Emergency treatment: IV calcium gluconate (10 mL of 10% over 2-3 min) to stabilise myocardium, followed by IV insulin 10 units with 50 mL of 50% dextrose to shift K⁺ intracellularly. Dialysis if refractory.
• Cushing's Syndrome: Diagnosis and Physiology: Caused by excess cortisol. Screening: 1 mg overnight dexamethasone suppression test (cortisol >1.8 mcg/dL indicates failure to suppress). Confirm with 24-hour urinary free cortisol or midnight salivary cortisol. First-line treatment: transsphenoidal adenomectomy for pituitary-dependent disease.
• Diabetes Insipidus: Water Deprivation Test: After water deprivation, if urine osmolality remains <300 mOsm/kg and plasma osmolality >295 mOsm/kg, administer desmopressin. In central DI, urine osmolality rises >50%; in nephrogenic DI, <50%. Polyuria >40 mL/kg/day with dilute urine.
• Cardiac Output: Fick Principle: Cardiac output (CO) = oxygen consumption / (arterial O₂ content - mixed venous O₂ content). Normal CO: 4-8 L/min. In heart failure, reduced ejection fraction <40% and elevated pulmonary capillary wedge pressure >15 mmHg.
• Renal Function: GFR and Creatinine Clearance: GFR estimated by Cockcroft-Gault: CrCl (mL/min) = [(140 - age) × weight (kg)] / (72 × serum creatinine) × 0.85 for females. CKD stages: Stage 3 GFR 30-59, Stage 4 GFR 15-29, Stage 5 <15 mL/min/1.73m².
• Hypothalamic-Pituitary-Adrenal Axis: ACTH Stimulation Test: For suspected adrenal insufficiency: give 250 mcg synthetic ACTH (cosyntropin) IV. Normal response: cortisol >18 mcg/dL at 30 or 60 min. In primary AI, baseline cortisol low with high ACTH; in secondary AI, low ACTH and blunted response.
• Anion Gap: Calculation and Causes: Anion gap = Na⁺ - (Cl⁻ + HCO₃⁻). Normal 8-12 mmol/L. Elevated gap (>12) in MUDPILES: Methanol, Uremia, DKA, Paraldehyde, Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates. Non-gap acidosis from diarrhea or renal tubular acidosis.

Common Traps in Physiology Questions

• Confusing respiratory compensation for metabolic acidosis with primary respiratory disorder: always use Winter's formula before diagnosing mixed acid-base disorder.
• Assuming all hyperkalaemia with ECG changes requires immediate calcium: only if ECG changes present (e.g., wide QRS) or K⁺ >6.5 mmol/L.
• Mistaking nephrogenic for central diabetes insipidus: desmopressin response is key — if urine osmolality does not rise, it is nephrogenic.
• Forgetting to correct for albumin when calculating anion gap: for every 1 g/dL drop in albumin, add 2.5 to the calculated gap.
• Thinking Cushing's syndrome is always exogenous: always check for pituitary or adrenal adenoma with dexamethasone suppression and ACTH levels.
• Overlooking that in Addisonian crisis, hyponatraemia and hyperkalaemia occur together with hypoglycaemia: treat with IV hydrocortisone 100 mg stat and normal saline.

How to Revise Physiology for the FMGE

Focus on integrated physiological responses to clinical scenarios: acid-base, electrolytes, and endocrine axes. Questions are often based on a single lab value or ECG strip, requiring immediate recognition (e.g., hyperkalaemia with wide QRS). Prioritise memorising normal ranges (e.g., pH 7.35-7.45, HCO₃⁻ 22-26, PaCO₂ 35-45 mmHg) and compensatory rules. Practice ABG interpretation and apply Winter's formula, anion gap, and osmolar gap. For cardiovascular, know Frank-Starling curve, ejection fraction, and heart failure classification (NYHA). For renal, master GFR calculation and CKD staging. Expect direct clinical application: 'A patient with vomiting and hypoventilation — what is the acid-base disorder?' (metabolic alkalosis with respiratory compensation). Use mnemonics (MUDPILES, RTA types) and focus on conditions commonly tested: DKA, SIADH, diabetes insipidus, Cushing's, Addison's, and thyrotoxicosis.

Practise it: MedLumen has 50 Physiology questions for the FMGE, each with a full explanation and references.