Basic Sciences Applied to Surgery
This domain tests whether you can link mechanism to the bedside: why a local anaesthetic failed, which electrolyte explains a postoperative arrhythmia, why a wound dehisced at day 7, and which organism is likely in a given infection. Most stems are two-step - a clinical scenario that asks for the underlying basic-science explanation.
Regional anatomy that is regularly examined
- Inguinal canal: anterior wall external oblique aponeurosis; posterior transversalis fascia plus conjoint tendon; floor inguinal ligament plus lacunar ligament; roof internal oblique and transversus abdominis. Direct hernia passes medial to the inferior epigastric vessels, through Hesselbach's triangle.
- Calot's triangle: cystic duct, common hepatic duct, inferior liver edge; its content is the cystic artery - the landmark for safe cholecystectomy.
- Recurrent laryngeal nerve: the right loops under the right subclavian artery, the left under the aortic arch; both then pass in the tracheo-oesophageal groove. A non-recurrent right nerve occurs with an aberrant right subclavian artery.
- Femoral triangle: sartorius laterally, adductor longus medially, inguinal ligament above; nerve, artery, vein, empty canal from lateral to medial.
Physiology, pharmacology and pathology
- Total body water is about 60% of weight; extracellular 20%, plasma 5%. Shifts follow Starling forces; inflammation raises capillary permeability, so oedema forms at lower hydrostatic pressure.
- Local anaesthetics are weak bases; the uncharged fraction crosses the membrane. Potency tracks lipid solubility, onset tracks pKa (closer to 7.4 is faster), duration tracks protein binding.
- Wound healing: haemostasis, inflammation, proliferation, remodelling. Collagen III is replaced by I; strength is about 10% at 1 week, 30% at 3 weeks and at most 80%.
- Surgical site infection: Staphylococcus aureus, coagulase-negative staphylococci, Escherichia coli. Prophylaxis is given 60 minutes before incision (120 for vancomycin) and re-dosed in long operations.
- Virchow's triad (stasis, endothelial injury, hypercoagulability) explains postoperative deep vein thrombosis.
Worked example
A 24-year-old man has a large buttock abscess. Infiltration with lidocaine produces almost no anaesthesia. Reasoning: lidocaine pKa is 7.9, so at pH 7.4 only about a quarter is uncharged; pus at pH near 6.0 shifts almost all drug to the charged form that cannot cross the nerve membrane, and hyperaemia also washes drug away. The right move is incision and drainage first, then infiltrate (or use buffered lidocaine) - not simply to inject a larger volume.
Traps that lose marks
Confusing pKa with potency; assuming penicillin allergy always precludes cephalosporins (cross-reactivity is low, under 2%); forgetting the vitamin K cycle when a jaundiced patient bleeds; attributing a postoperative sodium rise to SIADH when the cause is usually water deficit.