Surgical Microbiology & Immunology Flashcards

What is the molecular mechanism of MRSA resistance to beta-lactam antibiotics?

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.

Which cytokines are the primary endogenous pyrogens responsible for fever in sepsis?

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.

What initiates the classical pathway of complement activation?

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.

Which antibiotic is currently the first-line treatment for severe Clostridioides (Clostridium) difficile infection?

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.

Which is the most significant risk factor for ventilator-associated pneumonia (VAP) in postoperative surgical patients?

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.

Which type of allograft rejection occurs within minutes to hours of transplantation and is mediated by pre-formed donor-specific antibodies?

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.

Which organism is the most common cause of catheter-associated urinary tract infections (CAUTI) in hospitalised surgical patients?

Answer: Escherichia coli

Escherichia coli remains the most common pathogen in CAUTI, originating from periurethral colonisation with gut flora facilitated by the catheter.