FRCS Surgical Microbiology & Immunology 2 — Questions and Answers
Question 1: What is the molecular mechanism of MRSA resistance to beta-lactam antibiotics?
- Production of extended-spectrum beta-lactamase (ESBL)
- Expression of an altered penicillin-binding protein (PBP2a) encoded by mecA (Correct answer)
- Overexpression of efflux pumps reducing intracellular antibiotic concentration
- Reduced outer membrane permeability preventing antibiotic entry
Correct answer: Expression of an altered penicillin-binding protein (PBP2a) encoded by mecA
MRSA resistance is conferred by the mecA gene encoding PBP2a, an altered penicillin-binding protein with very low affinity for all beta-lactam antibiotics.
Question 2: Which cytokines are the primary endogenous pyrogens responsible for fever in sepsis?
- IL-4 and IL-13
- IL-10 and TGF-beta
- TNF-alpha and IL-1 (Correct answer)
- IL-6 and IL-8 exclusively
Correct answer: TNF-alpha and IL-1
TNF-alpha and IL-1 are the primary endogenous pyrogens released by macrophages that act on the hypothalamus via prostaglandin E2 to raise the temperature set-point.
Question 3: What initiates the classical pathway of complement activation?
- Mannose residues on microbial surfaces binding MBL
- C1q binding to antigen-antibody complexes (Correct answer)
- Spontaneous hydrolysis of C3 in plasma
- Factor B binding to microbial surfaces
Correct answer: C1q binding to antigen-antibody complexes
The classical pathway is initiated when C1q binds to Fc regions of IgG or IgM antibodies already bound to antigen, activating C1r and C1s.
Question 4: Which antibiotic is currently the first-line treatment for severe Clostridioides (Clostridium) difficile infection?
- IV metronidazole
- Oral amoxicillin-clavulanate
- Oral vancomycin or fidaxomicin (Correct answer)
- IV ciprofloxacin
Correct answer: Oral vancomycin or fidaxomicin
Oral vancomycin or fidaxomicin is the recommended first-line treatment for C. difficile infection; oral vancomycin achieves very high intraluminal concentrations without systemic absorption.
Question 5: Which is the most significant risk factor for ventilator-associated pneumonia (VAP) in postoperative surgical patients?
- Age over 65 years
- Mechanical ventilation via endotracheal tube (Correct answer)
- Body mass index greater than 35
- Pre-existing diabetes mellitus
Correct answer: Mechanical ventilation via endotracheal tube
Mechanical ventilation bypasses the natural upper airway defences and allows micro-aspiration of oropharyngeal secretions around the cuff, making it the primary risk factor for VAP.
Question 6: Which type of allograft rejection occurs within minutes to hours of transplantation and is mediated by pre-formed donor-specific antibodies?
- Acute cellular rejection
- Chronic allograft nephropathy
- Hyperacute rejection (Correct answer)
- Graft-versus-host disease (GvHD)
Correct answer: Hyperacute rejection
Hyperacute rejection is caused by pre-formed donor-specific antibodies (often against ABO or HLA antigens) that immediately activate complement on graft endothelium, causing thrombosis.
Question 7: Which organism is the most common cause of catheter-associated urinary tract infections (CAUTI) in hospitalised surgical patients?
- Staphylococcus aureus
- Escherichia coli (Correct answer)
- Pseudomonas aeruginosa
- Candida albicans
Correct answer: Escherichia coli
Escherichia coli remains the most common pathogen in CAUTI, originating from periurethral colonisation with gut flora facilitated by the catheter.
FRCS Surgical Microbiology & Immunology 2
What is the molecular mechanism of MRSA resistance to beta-lactam antibiotics?