Para-clinical Sciences: Pathology, Pharmacology, Microbiology, Forensic Medicine and Community Medicine
This block rewards pattern recognition plus one calculation or classification step. Pathology asks necrosis versus apoptosis, grading versus staging; pharmacology asks kinetics and interactions; microbiology asks media, stains and virulence factors; forensic medicine asks timelines; community medicine asks study design and screening arithmetic.
Core rules and high-yield facts
- Pathology: coagulative necrosis (ischaemia, architecture preserved), caseous (tuberculosis), liquefactive (brain, abscess), fat (pancreas), fibrinoid (vasculitis). Grading describes differentiation; staging describes spread and drives prognosis.
- Pharmacology: zero-order (saturated) kinetics applies to phenytoin, ethanol and high-dose salicylate - a fixed amount is eliminated per unit time, so small dose increments cause large level jumps. First-order drugs eliminate a constant fraction.
- Microbiology: TCBS agar for Vibrio cholerae, Löwenstein–Jensen for Mycobacterium tuberculosis, blood agar for haemolysis patterns, Robertson's cooked meat for anaerobes.
- Community medicine: sensitivity and specificity are properties of the test; predictive values depend on prevalence.
Worked example
A 2×2 table gives 90 true positives, 10 false negatives, 180 false positives and 720 true negatives out of 1,000. Sensitivity = 90/100 = 90%; specificity = 720/900 = 80%; positive predictive value = 90/270 = 33%. Reasoning: sensitivity uses the diseased column, specificity the healthy column, and PPV uses the test-positive row. Because prevalence here is only 10%, most positives are false - the classic trap when a stem reports a "highly sensitive test" in a low-prevalence population.
Traps that cost marks
- Confusing incidence (new cases/time) with prevalence (all cases at a point).
- Sterilisation kills spores; disinfection does not. Boiling is disinfection.
- Rigor mortis begins in 2–4 hours, is complete by 12 and passes off by 24–48 hours; do not quote a single figure without the qualifier.
- Enzyme inducers (rifampicin, phenytoin, carbamazepine) lower drug levels; inhibitors (ketoconazole, erythromycin) raise them.