Chemical pathology (high yield)

A 64-year-old patient with small-cell lung carcinoma presents with euvolemic hyponatremia. Laboratory results demonstrate: Serum Na+ 119 mmol/L, Serum Osmolality 248 mOsm/kg (RI: 275–295), Urine Osmolality 540 mOsm/kg, Urine Na+ 68 mmol/L, normal thyroid function, and normal morning cortisol. Which disorder best explains this profile?

Syndrome of inappropriate antidiuretic hormone secretion

A routine outpatient serum biochemistry panel shows: Na+ 140 mmol/L, K+ 7.4 mmol/L, Cl- 103 mmol/L, HCO3- 24 mmol/L, Creatinine 78 µmol/L, eGFR >90 mL/min/1.73m², and Hemolysis Index = 12 (normal). The patient is completely asymptomatic with a normal ECG. A contemporaneous FBC reveals WBC 245 × 10⁹/L (chronic lymphocytic leukemia) and Platelets 890 × 10⁹/L. What is the most likely cause of the elevated potassium?

Extreme leukocytosis (>100 × 10⁹/L) and thrombocytosis (>500 × 10⁹/L) cause in vitro potassium release from fragile leukocytes

Osmolar gap calculation

2(Na+) + Glucose + Urea

Which endogenous biomarker provides the most accurate non-invasive assessment of GFR independent of muscle mass and dietary meat/creatine intake?

Cystatin C

An asymptomatic 21-year-old medical student notices mild scleral icterus following a viral upper respiratory illness and exam-related fasting. LFTs show: Total Bilirubin 44 µmol/L, Conjugated Bilirubin 4 µmol/L, ALT 22 U/L, AST 24 U/L, ALP 68 U/L, GGT 19 U/L, normal FBC, normal reticulocyte count, and normal haptoglobin. What is the molecular basis of this condition?

Gilbert syndrome

A 38-year-old female presents with weight gain, cold intolerance, fatigue, and dry skin. Thyroid function tests reveal: TSH 42.5 mIU/L (RI: 0.40–4.00), Free T4 5.2 pmol/L (RI: 10.0–20.0), Free T3 2.8 pmol/L (RI: 3.5–6.5), and markedly elevated Anti-Thyroid Peroxidase (Anti-TPO) antibodies (>1,000 IU/mL). What is the diagnosis?

Primary hypothyroidism due to Hashimoto’s

A 44-year-old female with truncal obesity, violaceous abdominal striae, hypertension, and proximal muscle weakness fails to suppress her morning serum cortisol following a 1 mg overnight dexamethasone suppression test (post-dexamethasone cortisol = 380 nmol/L; normal suppression <50 nmol/L). Her baseline plasma ACTH is undetectable (<1.1 pmol/L). What is the most likely etiology?

Cushing’s syndrome

A 63-year-old female is found on routine biochemistry to have: Albumin-adjusted Calcium 2.88 mmol/L (RI: 2.15–2.55), Phosphate 0.68 mmol/L (RI: 0.80–1.50), Parathyroid Hormone (PTH) 14.8 pmol/L (RI: 1.6–6.9), 25-hydroxyvitamin D 68 nmol/L, and normal renal function. Which diagnosis explains this biochemical triad?

Primary hyperparathyroidism