Pharmacology & micro for MRCS Part A

What the paper tends to test

• Questions typically test local anaesthetic maximum doses and the recognition and treatment of local anaesthetic systemic toxicity with lipid emulsion.
• Expect anticoagulant questions on the mechanism and reversal of warfarin, heparins and the direct oral anticoagulants, and on stopping or bridging them before surgery.
• Antibiotic questions ask which agent covers which organism, when prophylaxis is given, and which drugs most often precipitate Clostridioides difficile infection.
• Perioperative drug questions ask what to do with steroids, insulin, antiplatelets, ACE inhibitors and the combined oral contraceptive before an operation.
• Microbiology lines test the organisms behind specific infections — Staphylococcus aureus in surgical site infection, Pseudomonas in burns and ventilated patients, Clostridium perfringens in gas gangrene — and the sterilisation method for a given instrument.
• Imaging questions compare modalities for a clinical problem, cover the risks of iodinated and gadolinium contrast, and use effective radiation dose as the discriminator.
• Statistics questions need a short calculation — sensitivity, specificity, predictive values, number needed to treat — or the right test for a type of data; audit is tested against research and service evaluation.

Classic traps and discriminators

• The maximum doses usually quoted are lidocaine 3 mg/kg plain and 7 mg/kg with adrenaline, and bupivacaine 2 mg/kg with or without adrenaline. A 1% solution contains 10 mg/ml, so 20 ml of 1% lidocaine is 200 mg — the arithmetic is where the marks are lost.
• Warfarin is reversed with vitamin K and, for major bleeding, four-factor prothrombin complex concentrate; unfractionated heparin with protamine, which only partly reverses low-molecular-weight heparin; dabigatran with idarucizumab; and the factor Xa inhibitors with andexanet alfa where available, otherwise prothrombin complex concentrate.
• Cephalosporins, fluoroquinolones, clindamycin and broad-spectrum penicillins are the antibiotics most associated with Clostridioides difficile infection; NICE NG199 gives oral vancomycin first line and oral fidaxomicin for a recurrence within 12 weeks.
• Autoclaving — steam under pressure, typically 134 °C for 3 minutes — sterilises; glutaraldehyde, ethylene oxide and low-temperature plasma are used for heat-sensitive equipment; prions survive standard autoclaving, so instruments used on a patient with suspected CJD are quarantined or destroyed.
• Sensitivity and specificity are properties of the test and do not change with prevalence; positive and negative predictive values do, so a screening test for a rare disease produces many false positives even when it is highly specific.
• The effective dose of a chest radiograph is about 0.02 mSv and of an abdominal and pelvic CT roughly 10 mSv — several hundred chest films and a few years of natural background radiation — which is the figure questions use to argue for ultrasound or MRI in the young and the pregnant.

Free revision notes

Pharmacology & micro questions, answered