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DR15.1-3 | Pyoderma — Graded Quiz
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A 3-year-old child develops widespread flaccid bullae and superficial erosions over the trunk, buttocks, and face following a febrile illness. The oral mucosa is spared. Nikolsky sign is positive. Blood culture is negative. What is the most likely diagnosis and its pathological mechanism?
Correct. SSSS in children: negative blood culture (bacteraemia rare), mucosal sparing (ET cleaves desmoglein-1, expressed in skin; desmoglein-3 protects mucosa), and a benign prognosis with antistaphylococcal antibiotics.
SSSS is caused by haematogenous spread of exfoliative toxin from a remote Staph aureus focus; the source (e.g., conjunctivitis, umbilicus) is not at the site of blistering. Mucosal sparing and a positive Nikolsky sign in a child with negative blood culture support SSSS over TEN.
Incorrect. TEN occurs in older children/adults following drug exposure, has mucosal involvement, and shows full-thickness epidermal necrosis. Bullous pemphigoid and pemphigus are autoimmune and rare in this age group with this clinical picture.
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A 55-year-old immunosuppressed renal transplant recipient develops sudden-onset bright-red, sharply demarcated erythema on the left leg, raised above the surrounding skin, with fever of 39°C and lymphadenopathy. Which organism is most likely, and what is first-line treatment?
Correct. The raised, sharply-marginated plaque on the leg is the hallmark of erysipelas caused by Strep pyogenes. Penicillin remains the drug of choice with excellent susceptibility.
Erysipelas (sharply demarcated, raised, hot erythema) is almost exclusively caused by Streptococcus pyogenes. Benzylpenicillin IV (for severe/hospitalised) or amoxicillin oral (mild) is first-line. Cloxacillin is appropriate for staphylococcal disease, not erysipelas.
Incorrect. The sharply raised, demarcated plaque is erysipelas — Strep pyogenes. Cloxacillin/MRSA regimens are not first-line here.
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A patient on long-term doxycycline for MRSA furunculosis develops increasing photosensitivity and oesophageal discomfort. Which instruction would be MOST important to prevent the oesophageal complication?
Correct. Oesophageal ulceration from doxycycline is caused by direct mucosal contact. The tablet must be taken with plenty of water and the patient must not lie down for 30 minutes.
Doxycycline ADRs: photosensitivity (use sunscreen, protective clothing), oesophageal ulceration (prevent by taking with a full glass of water and remaining upright), GI upset, and teratogenicity/tooth discolouration in pregnancy and children <8 years.
Incorrect. Milk and antacids chelate doxycycline (with calcium/magnesium), reducing absorption — this should be AVOIDED. Dose reduction compromises MRSA treatment. IV bypass is impractical for outpatient use.
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A 35-year-old man presents with a fluctuant, tender 3 cm abscess on the left thigh. He has no fever, no surrounding cellulitis, and is otherwise well. He has no drug allergies. What is the most appropriate management?
Correct. I&D is the definitive treatment for a fluctuant abscess. Current evidence (including NEJM trials) supports I&D alone without routine antibiotics for uncomplicated abscesses in immunocompetent patients.
For a simple, uncomplicated cutaneous abscess in a healthy patient, incision and drainage alone is the treatment of choice. Adjunctive antibiotics are added only if there are systemic features, surrounding cellulitis, immunosuppression, or failed I&D.
Incorrect. Antibiotics alone cannot drain pus. Warm compresses are appropriate only for early non-fluctuant furuncles. IV vancomycin is reserved for complicated MRSA infections, not a simple uncomplicated abscess.
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A 60-year-old diabetic is admitted with rapidly spreading leg pain, fever 38.8°C, and skin that appears only mildly erythematous. On palpation you feel crepitus along the medial thigh. CT scan shows gas in the fascial planes. What is the immediate management priority?
Correct. NSTI is a life-threatening emergency. The triad of crepitus, gas on CT, and disproportionate pain in a diabetic mandates immediate surgical referral. Broad-spectrum IV antibiotics (covering Gram-positives, Gram-negatives, and anaerobes) are given concurrently but do NOT replace surgery.
Necrotising soft-tissue infection is a surgical emergency. Antibiotics alone cannot penetrate ischaemic necrotic tissue. Survival depends on early wide surgical debridement; mortality increases sharply with every hour of delay.
Incorrect. Antibiotics alone are insufficient because they cannot penetrate necrotic ischaemic tissue. Needle aspiration and hyperbaric oxygen are adjuncts at best; surgery is the non-negotiable first priority.
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Which combination correctly matches the pyoderma with its anatomical depth of infection?
Correct. This depth hierarchy is central to understanding why progressively deeper infections require progressively more aggressive treatment — from topical antibiotics (impetigo) to I&D (furuncle/carbuncle) to radical surgery (NSTI).
Classifying pyoderma by depth: impetigo (epidermis) → folliculitis (hair follicle ostium) → furuncle (deep follicular abscess into dermis) → carbuncle (coalescing furuncles through subcutis) → NSTI (fascia and muscle).
Incorrect. The depth hierarchy determines treatment intensity. Review: impetigo (epidermis), folliculitis (follicular ostium), furuncle (deep follicle/dermis), carbuncle (subcutaneous), NSTI (fascia/muscle).
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A patient receiving oral co-trimoxazole for CA-MRSA furunculosis develops a maculopapular rash on day 7. On review, the patient also has oral ulcers and conjunctival injection. What is the MOST important immediate action?
Correct. Mucosal involvement (oral ulcers + conjunctival injection) alongside a new rash on co-trimoxazole is SJS until proven otherwise. Stop the drug immediately, provide supportive care, and switch to an alternative MRSA agent (doxycycline, clindamycin per susceptibility).
Co-trimoxazole is a well-known cause of Stevens-Johnson syndrome (SJS), characterised by mucocutaneous involvement (targetoid/purpuric lesions, mucosal erosions, conjunctivitis). Immediate drug withdrawal is mandatory; continuing the drug dramatically worsens outcomes.
Incorrect. A rash with oral ulcers and conjunctival involvement while on co-trimoxazole indicates a potentially life-threatening drug reaction (SJS/TEN spectrum). Continuing, dose-halving, or adding steroids without withdrawal are dangerous. Immediate drug withdrawal is the priority.
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Recurrent furunculosis (>3 episodes per year) in a patient with no identifiable immunodeficiency most often reflects carriage of Staphylococcus aureus at which site?
Correct. ~30% of the general population are nasal carriers of Staph aureus. Recurrent furunculosis often results from autoinoculation from the anterior nares to skin, especially in warm/humid conditions or with nasal-picking.
The anterior nares are the primary reservoir of Staph aureus carriage. Nasal decolonisation with intranasal mupirocin (± chlorhexidine body wash) is used in recurrent furunculosis to interrupt the cycle of autoinoculation.
Incorrect. While Staph aureus can colonise multiple sites, the anterior nares are the primary carriage site and the usual source in recurrent furunculosis.
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A clinician is evaluating a spreading leg infection and must decide between erysipelas and cellulitis. Which SINGLE bedside finding most reliably separates the two?
Correct. The palpably raised, clearly demarcated, 'plaque-like' border of erysipelas reflects its superficial upper-dermal plane. This bedside sign is what drives the differentiation and confirms the Streptococcal aetiology.
The sharp, palpably raised margin is the single most reliable clinical discriminator of erysipelas from cellulitis. Both can cause fever, lymphangitis, and lymphadenopathy. Both can occur at any age.
Incorrect. Fever and lymphangitis occur in both conditions. Age is not a reliable discriminator. Border sharpness and elevation are the definitive distinguishing features.
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A patient is being started on oral cloxacillin for a deep-seated furunculosis. Which adverse effect profile correctly describes cloxacillin?
Correct. Cloxacillin's ADR profile is that of the penicillin class: allergy reactions, GI upset, and rare hepatitis. Always ask about penicillin allergy before prescribing; in IgE-mediated allergy, use a cephalosporin with caution (10% cross-reactivity) or switch to vancomycin.
Cloxacillin is a penicillinase-resistant penicillin. ADRs: penicillin-class hypersensitivity (ranging from rash to anaphylaxis), GI intolerance, and rare cholestatic hepatitis (especially with flucloxacillin, a closely related agent).
Incorrect. Nephrotoxicity/ototoxicity + drug monitoring describes aminoglycosides (gentamicin, amikacin). QT prolongation + tendon rupture describes fluoroquinolones. Photosensitivity + tooth discolouration describes doxycycline.
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