Basic Sciences Applied to Surgery
The Basic Sciences domain tests whether you can convert anatomy, physiology, pathology, microbiology and pharmacology into operative decisions - not whether you can recite lists. Questions cluster around surgical anatomy, fluid and electrolyte physiology, wound biology, common pathogens and drug dosing.
Anatomy worth memorising
- Indirect inguinal hernia passes lateral to the inferior epigastric vessels through the deep ring; a direct hernia passes medial to them through Hesselbach's triangle.
- Calot's triangle is bounded by the cystic duct, the common hepatic duct and the inferior surface of the liver; the cystic artery normally runs within it.
- Femoral triangle, lateral to medial: femoral nerve, artery, vein, empty space, lymphatics (NAVEL).
Physiology and pharmacology
- Total body water is about 60% of body weight; intracellular 40%, extracellular 20% (interstitial 15%, plasma 5%).
- The 4-2-1 rule gives hourly maintenance fluid: 4 mL/kg for the first 10 kg, 2 mL/kg for the next 10 kg, 1 mL/kg thereafter.
- Lidocaine maximum 3 mg/kg plain and 7 mg/kg with adrenaline; bupivacaine 2 mg/kg. The commonest cause of local anaesthetic systemic toxicity is inadvertent intravascular injection and it is treated with 20% lipid emulsion.
Worked example
A 70 kg man needs 1% lidocaine without adrenaline for a scalp laceration. Maximum dose = 3 mg/kg × 70 = 210 mg. A 1% solution contains 10 mg/mL, so the ceiling is 21 mL. His maintenance fluid by 4-2-1 = (4×10) + (2×10) + (1×50) = 110 mL/h.
Traps
Swapping intracellular and extracellular volumes; quoting 4-2-1 as a daily total rather than an hourly rate; memorising maximum local anaesthetic doses in millilitres instead of mg/kg, which fails when the concentration changes; forgetting that third-space losses require additional replacement beyond maintenance.