MI5.1-5 | Musculoskeletal, Skin & Soft-Tissue Infections — Glossary

Phospholipase C (lecithinase) produced by C. perfringens type A; destroys cell membranes of erythrocytes, leucocytes, and myocytes; primary virulence factor in gas gangrene.

Microorganism that grows in the absence of oxygen; lacks catalase and/or superoxide dismutase; thrives in devitalised, ischaemic tissue with low redox potential.

Semi-quantitative scale (1+–6+) of M. leprae density in a slit-skin smear; does not change rapidly with treatment; monitors bacillary load.

Most common anaerobic Gram-negative rod in clinical specimens; encapsulated; resistant to penicillin (beta-lactamase); causes abdominal/pelvic abscesses.

Acute infection of dermis and subcutaneous tissue; poorly marginated erythema, warmth, oedema; usually S. aureus or S. pyogenes; no gas (differentiates from NF).

Phenazine dye antibiotic used in MDT for MB leprosy; binds to guanine bases of DNA; anti-inflammatory; causes red-brown skin discolouration and xerosis.

Group of keratinophilic fungi (Trichophyton, Microsporum, Epidermophyton) that infect only keratinised tissues (skin, hair, nails); identified by KOH hyphae and SDA culture.

Immune complex deposition reaction in lepromatous/borderline lepromatous leprosy; tender erythematous skin nodules + fever + iritis; treated with clofazimine or thalidomide.

Superficial skin infection involving dermis and superficial lymphatics; caused by S. pyogenes; sharply demarcated, raised, red, hot, tender plaque; treat with penicillin.

Serine protease produced by S. aureus phage group II strains; cleaves desmoglein 1 in superficial epidermis → bullous impetigo and staphylococcal scalded skin syndrome.

Clear band of uninvolved collagen separating the epidermis from the dermal granulomatous infiltrate; specific to lepromatous leprosy histopathology.

Superficial epidermal infection; non-bullous (S. pyogenes, honey-coloured crusts) or bullous (S. aureus exfoliatin toxin, flaccid bullae); treated with topical mupirocin or penicillin.

New bone formed around a sequestrum in chronic osteomyelitis by the elevated periosteum; visible on X-ray as a collar of new cortical bone.

Acute upgrading of cell-mediated immunity in borderline leprosy; manifest as sudden oedema and erythema of existing lesions + acute nerve damage; treated with prednisolone.

Multibacillary leprosy pole with weak Th1, dominant Th2; numerous skin lesions; foam cells (Virchow cells); Grenz zone; BI 4+–6+; leonine facies.

Loss of eyebrows and eyelashes; characteristic of lepromatous leprosy due to bacillary infiltration of hair follicles in the supraorbital region.

Thick-walled, brown, septate sclerotic cells in tissue; pathognomonic of chromoblastomycosis caused by dematiaceous (dark-walled) fungi like Fonsecaea pedrosoi.

Percentage of solid-staining (viable) M. leprae in a slit-skin smear; falls rapidly with effective MDT — useful early treatment response marker.

Methicillin-resistant Staphylococcus aureus; mecA gene encodes PBP2a (low affinity for beta-lactams); CA-MRSA causes SSTIs; HA-MRSA causes hospital infections; treat with vancomycin.

Leprosy classification: >5 skin lesions OR BI positive; treated with rifampicin + clofazimine + dapsone × 12 months.

Chronic subcutaneous infection with triad: painless swelling + sinus tracts + coloured granules; actinomycetoma (Nocardia, Actinomyces) or eumycetoma (Madurella).

Destruction of muscle tissue; in gas gangrene, caused by C. perfringens alpha and theta toxins; characterised by gas production, rapid progression, systemic toxaemia.

Plate test for C. perfringens alpha toxin: egg-yolk agar with anti-alpha toxin on one half; opaque zone of lecithinase activity forms only on uninhibited side.

Rapidly progressive, life-threatening infection of the fascia and subcutaneous tissue; type I (polymicrobial, anaerobes + facultatives), type II (Group A Strep).

Infection of bone; haematogenous (children: metaphysis; adults: vertebrae) or contiguous; S. aureus most common overall; Salmonella in sickle cell disease.

Leprosy classification: ≤5 skin lesions, slit-skin smear BI negative; treated with rifampicin + dapsone × 6 months.

Vertebral osteomyelitis caused by M. tuberculosis; affects the disc space and adjacent vertebral bodies; may cause spinal cord compression and gibbus deformity.

Sterile inflammatory arthritis triggered by prior urogenital (Chlamydia) or gastrointestinal (Salmonella, Shigella, Campylobacter) infection; joint culture is negative.

Rickettsia tsutsugamushi infection via mite bite; characteristic eschar (painless black necrotic crust at bite site) + fever + maculopapular rash; treated with doxycycline.

Bacterial infection of a joint; emergency — synovial proteases destroy cartilage within 4–6 hours; requires joint aspiration, culture, and drainage.

Dead bone in chronic osteomyelitis; avascular, devitalised cortical bone that acts as a reservoir for bacteria, preventing antibiotic penetration.

Diagnostic technique for leprosy: a slit in anaemic skin (pinched) yields dermal fluid; spread on slide, ZN-stained, graded for BI.

Subcutaneous mycosis caused by Sporothrix schenckii; classical form: nodular lymphangitis after thorn prick; cigar-shaped yeast in tissue; treated with itraconazole.

Zinc metalloprotease toxin of C. tetani; travels retrograde to anterior horn cells and cleaves synaptobrevin, blocking GABA/glycine release → spastic paralysis.

Paucibacillary leprosy pole with strong Th1 (cell-mediated) immunity; 1–5 anaesthetic hypopigmented patches; well-formed granulomas; BI negative.

Cytological test from vesicle base; shows multinucleated giant cells (Cowdry type A intranuclear inclusions) in HSV or VZV infections.

Lipid-laden macrophages packed with M. leprae in lepromatous leprosy; unable to kill bacilli due to absent Th1 CMI response.