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

A 45-year-old man presents with a 5-day history of a puncture wound on his foot with progressive pain, bronze-green discolouration of the overlying skin, and a sweet, foul odour. X-ray shows gas in soft tissues. Gram stain of wound exudate reveals Gram-positive rods WITHOUT pus cells, and culture on blood agar under anaerobic conditions shows double-zone haemolysis. Which toxin is primarily responsible for the rapid tissue destruction seen in this organism?

A 28-year-old unvaccinated man presents with a 2-day history of difficulty opening his mouth (trismus), neck stiffness, and painful spasms of the back triggered by noise. Vital signs: temp 37.8°C. The wound entry site was a minor abrasion sustained 12 days ago. Cerebrospinal fluid analysis is NORMAL. Which of the following BEST explains why CSF is normal in this condition?

A 7-year-old girl presents with fever and acute onset of severe pain and swelling of the right knee. She had a throat infection 2 weeks ago that resolved without antibiotic treatment. Joint aspiration yields 60,000 WBC/mm3 (95% neutrophils), low glucose, high protein. Gram stain shows Gram-positive cocci in chains. Blood culture is positive for the same organism. Which is the MOST likely organism, and what is its mechanism of joint damage?

An HIV-positive patient (CD4 count 85 cells/uL) develops a chronic swelling of the right knee with multiple draining sinuses. Aspirated fluid shows large, thick-walled spherical cells that reproduce by broad-based budding. Histopathology reveals mixed granulomatous and suppurative inflammation. Which organism MOST likely causes this presentation of fungal osteomyelitis/arthritis?

A 60-year-old man who was treated for pulmonary tuberculosis 5 years ago presents with back pain, fever, and progressive lower limb weakness. MRI shows vertebral body destruction at L2–L3 with a paravertebral cold abscess. There is no local heat or tenderness over the spine. Which feature BEST explains why the abscess remains cold in this infection?

A 4-year-old child presents with multiple vesicles, pustules, and honey-coloured crusts on the face and around the nares. The mother also notes some large, flaccid bullae on the trunk. Nikolsky sign is positive over the bullous lesions. Which statement BEST explains the pathogenesis of the bullae in this patient?

A 40-year-old patient presents with numerous, symmetric, poorly defined infiltrated plaques and nodules over the face, ears, and extremities. Sensation is intact over all lesions. Nasal scrapings reveal numerous acid-fast bacilli on Ziehl-Neelsen stain. Slit-skin smear shows a Bacteriological Index (BI) of +5. Lepromin test is negative. Which cell-mediated immune defect MOST accurately characterises this patient's immune status in the context of leprosy?

A dermatology resident biopsies a skin lesion from a patient with suspected leprosy. Haematoxylin and eosin staining shows a clear band of uninvolved collagen between the epidermis and the underlying granulomatous infiltrate filled with foamy macrophages containing numerous acid-fast bacilli. What is this histopathological finding called, and in which type of leprosy is it characteristically found?

A clinician is assessing a patient with skin lesions and must determine whether the lesion is infective or non-infective before ordering investigations. Which combination of clinical features would MOST strongly suggest an INFECTIVE aetiology rather than a non-infective dermatological condition?

A 35-year-old returned traveller from Sub-Saharan Africa develops a painless, rapidly enlarging ulcer on the forearm with undermined, irregular edges and overlying skin that is serosanguineous but not frankly purulent. The lesion is painless despite extensive necrosis. Biopsy shows fat cell necrosis with ghost outlines of adipocytes and no significant inflammatory infiltrate. AFB stain is positive. Which organism and its key virulence mechanism BEST accounts for this presentation?

A 55-year-old man with poorly controlled diabetes mellitus develops a chronic, non-healing ulcer on his right foot after a minor injury. Over 6 weeks, he develops fever, bone pain, and imaging shows periosteal elevation with a sequestrum surrounded by involucrum at the 1st metatarsal. Blood culture grows Gram-positive cocci in clusters. Bone biopsy confirms osteomyelitis. Which of the following BEST explains how a sequestrum forms in chronic osteomyelitis?

A 32-year-old patient with multibacillary leprosy on WHO-MDT for 8 months suddenly develops acutely tender, erythematous nodules over the forearms and trunk with high fever, arthralgia, iritis, and a positive urine for protein. Nerve conduction studies show acute peripheral neuropathy. What is this acute complication, and which immunological mechanism underlies it?