Biomedical Sciences: Structure, Function and Disease Mechanisms
What this domain tests
The biomedical sciences domain asks you to connect structure and function to drug action and disease. Expect homeostasis, fluid/electrolyte and acid-base regulation, cardiac and renal physiology, autonomic and receptor pharmacology foundations, inflammation and immunity, microbial structure and virulence, and core biochemistry (glycolysis, Krebs cycle, oxidative phosphorylation, enzyme kinetics). Questions often present a lab value or clinical vignette and ask which mechanism explains it.
Core rules
- Homeostasis is maintained by negative feedback; compensation rarely returns a value fully to normal.
- The anion gap is Na − (Cl + HCO3); normal is roughly 8–12 mmol/L. A raised gap suggests unmeasured anions (ketones, lactate, uremic acids, salicylate, methanol, ethylene glycol).
- Serum potassium is a poor marker of total body potassium; insulin and beta-2 agonists shift K intracellularly, while acidosis and cell lysis shift it out.
- Bacterial cell walls (peptidoglycan) are the target of beta-lactams and vancomycin; mycobacteria have mycolic acid; gram-negative outer membranes limit drug penetration.
- Enzyme velocity follows Michaelis–Menten kinetics; competitive inhibitors raise Km without changing Vmax, non-competitive inhibitors lower Vmax.
Worked example
A 24-year-old with type 1 diabetes presents with vomiting, glucose 28 mmol/L, pH 7.20, HCO3 10 mmol/L, Na 132, Cl 98, K 5.2. The anion gap is 132 − (98 + 10) = 24 mmol/L, consistent with ketoacidosis. The high serum K does not mean total body K is high: osmotic diuresis and vomiting deplete total body potassium, but insulin deficiency and acidosis shift K out of cells. Insulin therapy will drive K back into cells and can cause dangerous hypokalemia, so potassium must be monitored and replaced once urine output is established. The low pH also stimulates respiratory compensation (Kussmaul breathing), which lowers PaCO2.
Traps
Do not label a high-K serum in DKA as potassium excess; do not calculate compensation incorrectly; do not confuse respiratory alkalosis with metabolic acidosis; do not assume gram-positive and gram-negative organisms share susceptibility; do not memorize pathways without linking them to drug targets (e.g., metronidazole to anaerobic DNA, fluoroquinolones to topoisomerase).