Part I – Biomedical Science
Domain 1 – Biomedical Science
Part I – Biomedical Science tests the mechanisms beneath clinical findings: integrated anatomy and physiology, biochemistry, microbiology and immunology, general pathology, and foundational pharmacology. Expect stems asking which process, organism, mediator, or enzyme best explains an observation, not simply to name a disease.
Mechanism-first rules
- Cell injury: reversible injury shows swelling and fatty change; irreversible injury shows membrane rupture, mitochondrial permeability transition, and calcium influx. Apoptosis is caspase-dependent and non-inflammatory; necrosis is not.
- Inflammation: acute inflammation is neutrophil-predominant; chronic is macrophage/lymphocyte-predominant. IL-1, IL-6, and TNF-α drive fever and hepatic acute-phase reactants such as CRP and ferritin.
- Acid–base: calculate the anion gap (Na⁺ − [Cl⁻ + HCO₃⁻]) and check compensation with Winter's formula; an inappropriate PCO₂ means a mixed disorder.
- Microbiology: separate toxin-mediated from invasive disease. Cholera toxin ADP-ribosylates Gsα, raising cAMP; diphtheria toxin ADP-ribosylates EF-2, halting translation.
- Pharmacology: apply first-pass metabolism, volume of distribution, and half-life. Zero-order kinetics (phenytoin, ethanol) means a constant amount is eliminated per unit time, so small dose changes can cause toxicity.
Worked example
A traveler has profuse rice-water diarrhea and dehydration. Which mechanism fits? Cholera toxin persistently activates adenylate cyclase, increasing cAMP and driving CFTR chloride secretion while blocking sodium absorption. Reasoning: the stool is secretory and non-bloody, pointing to an enterotoxin rather than invasion. ETEC's heat-labile toxin also raises cAMP; Shigella invades mucosa and produces dysentery.
Mark-losing traps
- Confusing type I (IgE) with type IV (T-cell) hypersensitivity.
- Attributing a finding to a disease rather than the asked-for mechanism.
- Forgetting that reference ranges are population-based and not diagnostic thresholds.
- Mixing up vitamin cofactors (e.g., thiamine in pyruvate dehydrogenase) under time pressure.