Basic Medical Sciences: Mechanisms, Calculations and the Single Best Explanation
The Basic Medical Sciences domain tests whether you can apply physiology, biochemistry, pharmacology, pathology, microbiology and immunology to short clinical vignettes. Stems usually ask for a mechanism, a calculation, or the single best explanation for a laboratory pattern, so pure list recall is rarely enough.
Core rules
- Body water: total body water ≈ 0.6 × weight; ICF is two-thirds, ECF one-third; plasma is one-quarter of ECF (~3 L in a 70 kg adult).
- Clearance CL = (urine concentration × urine flow)/plasma concentration. Maintenance infusion rate = CL × target concentration / bioavailability.
- Half-life t½ = 0.693 × Vd/CL; after n half-lives the fraction remaining is 0.5^n (3 half-lives = 12.5%).
- Oxygen–haemoglobin curve shifts right (reduced affinity, better tissue delivery) with raised CO2, H+, temperature and 2,3-BPG; left with fetal haemoglobin, carboxyhaemoglobin and methaemoglobin.
- Anion gap = Na − (Cl + HCO3), normal 8–12 mmol/L; subtract ~2.5 from the expected gap for every 1 g/dL fall in albumin.
- Receptor map: α1 vasoconstriction and mydriasis; β1 chronotropy/inotropy; β2 bronchodilation and uterine relaxation; M2 cardiac slowing; M3 secretions and smooth-muscle contraction; nicotinic receptors at the neuromuscular junction.
- Competitive antagonists shift the log dose–response curve right without reducing Emax; non-competitive antagonists reduce Emax.
- Acute inflammation: neutrophils dominate the first 6–24 hours, macrophages after 24–48 hours; granulomas are Th1/IFN-γ driven.
Worked example
A 70 kg man is given 500 mg of a drug intravenously; the extrapolated peak concentration is 25 mg/L and the half-life is 4 hours. Vd = dose/C0 = 500/25 = 20 L, or 0.29 L/kg, consistent with distribution largely confined to extracellular fluid. CL = 0.693 × Vd/t½ = 0.693 × 20/4 ≈ 3.5 L/h. At 12 hours (three half-lives) 12.5% remains: 62.5 mg. If the target steady-state concentration is 10 mg/L and bioavailability is 1, the infusion rate is 3.5 × 10 ≈ 35 mg/h.
Traps
- Reporting Vd in L/kg and comparing it with plasma volume (3 L). Vd is an apparent volume and can exceed total body water.
- Missing a raised anion gap when albumin is low, or expecting a raised gap in hyperchloraemic acidosis.
- Confusing Km (substrate concentration at half Vmax) with Vmax; ethanol is zero-order because the enzyme is saturated at usual doses.
- Assuming high protein binding means small Vd - tissue binding matters more than plasma binding.
- Treating a left-shifted oxygen curve as beneficial; it impairs delivery.