Skin, soft tissue & Gram-positive infections flashcards
15 high-yield MRCP Part 1 cards from the Intellect Medicos Infectious Diseases course. Try to answer each question before you open it — then study them properly with spaced repetition, which brings each card back just before you would forget it.
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What is the commonest cause of cellulitis?
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Beta-haemolytic streptococci, mainly Streptococcus pyogenes (group A); Staphylococcus aureus less often.
Why? Cellulitis is infection of the dermis and subcutaneous tissue; erysipelas is the more superficial (upper dermis) form.
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What is the first-choice antibiotic for cellulitis in an adult without penicillin allergy (NICE)?
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Oral flucloxacillin for 5–7 days.
Why? It covers streptococci and S. aureus. Give IV only if the patient cannot take oral drugs or is severely unwell.
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A 55-year-old man with diabetes has a red, swollen thigh after a minor cut. He has pain far out of proportion to the skin changes, fever, and crepitus on palpation. What is the most important immediate management?
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Urgent surgical review for debridement, plus broad-spectrum IV antibiotics.
Why? This is necrotising fasciitis. Antibiotics alone cannot penetrate dead fascia; delay in debridement greatly increases mortality.
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Which toxin causes staphylococcal toxic shock syndrome?
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Toxic shock syndrome toxin-1 (TSST-1), a superantigen.
Why? Superantigens activate huge numbers of T cells at once, causing a cytokine storm with fever, hypotension and multi-organ failure.
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Besides removing the source (e.g. tampon), which antibiotic is often added in toxic shock syndrome to switch off toxin production?
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Clindamycin (a protein-synthesis inhibitor), alongside an anti-staphylococcal agent such as flucloxacillin or vancomycin.
Why? Clindamycin stops ribosomal toxin production even when bacteria are not dividing.
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Endocarditis caused by Streptococcus bovis (S. gallolyticus) should prompt which investigation?
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Colonoscopy — it is strongly associated with colorectal cancer.
Why? The organism enters the blood through abnormal colonic mucosa.
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A 6-year-old (unvaccinated) has a sudden high fever, drooling, a muffled voice, and is sitting forward in the tripod position. There is no cough. What must you NOT do, and what is the immediate priority?
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Do not examine the throat or lie the child flat; secure the airway urgently with senior ENT and anaesthetic help.
Why? This is acute epiglottitis; examining the throat can trigger complete airway obstruction.
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What is the WHO treatment for multibacillary (lepromatous) leprosy?
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Rifampicin + dapsone + clofazimine for 12 months.
Why? High bacterial load with poor cell-mediated immunity needs triple therapy for longer.
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Which form of leprosy has strong cell-mediated immunity, few lesions and early nerve thickening?
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Tuberculoid (paucibacillary) leprosy.
Why? Good T-cell immunity keeps bacterial numbers low but causes early, marked nerve damage (e.g. ulnar nerve claw hand, common peroneal foot drop).
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What genetic change makes Staphylococcus aureus methicillin resistant (MRSA)?
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The mecA gene, which codes for an altered penicillin-binding protein, PBP2a.
Why? PBP2a has low affinity for beta-lactams, so cell wall synthesis continues despite the antibiotic.
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How is an MRSA carrier decolonised before surgery?
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Mupirocin 2% nasal ointment plus chlorhexidine skin washes.
Why? Carriage is mainly in the nose and skin; suppression lowers the risk of post-operative MRSA infection.
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What is the first-line IV antibiotic for severe MRSA infection?
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Vancomycin (alternatives: teicoplanin, linezolid).
Why? Glycopeptides act on the cell wall at a site unaffected by PBP2a.
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A man who keeps tropical fish has a nodule on his finger that ulcerated, followed by more nodules spreading up his forearm along the lymphatics. What is the likely organism?
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Mycobacterium marinum ("fish tank granuloma").
Why? An atypical mycobacterium from water. Lymphatic ("sporotrichoid") spread is the classic sign.
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A 19-year-old had a sore throat a week ago. He now has fever, unilateral neck pain and swelling, and multiple cavitating lung nodules on CXR. Which organism is most likely?
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Fusobacterium necrophorum (Lemierre's syndrome).
Why? Oropharyngeal infection spreads to cause septic thrombophlebitis of the internal jugular vein, which throws septic emboli to the lungs.
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What is the first-line antibiotic for a human or cat/dog bite needing treatment (NICE)?
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Co-amoxiclav.
Why? It covers Pasteurella multocida (animal bites), Eikenella corrodens (human bites), streptococci, staphylococci and anaerobes.