A 28-year-old woman complains of gradually increasing pain around her right knee. Radiography demonstrates an eccentric, expansile, purely lytic lesion extending to the subarticular region of the distal femoral epiphysis. The lesion has a characteristic "soap-bubble" appearance. Histology reveals numerous uniformly distributed osteoclast-type giant cells among mononuclear stromal cells. Which diagnosis is most likely?

A 34-year-old woman receives a non-depolarizing neuromuscular blocker during surgery. At the end of the procedure, neostigmine is administered to reverse the blockade. Atropine is given simultaneously. Why is atropine administered with neostigmine?

A 35-year-old man is brought to the emergency department after a road-traffic collision. A 7-cm wound is present over the anteromedial tibia with visible bone and gross contamination by soil. Distal pulses are palpable and sensation is intact. Radiography confirms a comminuted tibial fracture communicating with the wound. After initial stabilization of airway, breathing, and circulation, the limb is covered with a sterile dressing. Which management step is most important in reducing the risk of deep infection?

A 46-year-old woman presents with a slowly enlarging painless mass in the thigh. MRI shows a deep-seated 9-cm soft-tissue mass. Biopsy demonstrates malignant spindle cells with marked pleomorphism but no identifiable line of differentiation. Which diagnosis best fits these findings?

A 24-year-old man presents with chronic lower-back and buttock pain for 18 months. Symptoms began insidiously, are worse during the second half of the night, and improve after exercise rather than rest. Examination shows reduced lumbar flexion and limited chest expansion. Pelvic radiography demonstrates bilateral sacroiliitis. Which diagnosis is most likely?

A 23-year-old sexually active woman presents with fever, migratory pain involving the wrists and ankles, and several pustular skin lesions over the distal extremities. Two days later she develops marked swelling of the right knee. Synovial fluid shows neutrophilic inflammation. Which organism is the most likely underlying cause?

A 45-year-old man undergoes abdominal surgery under general anesthesia. A neuromuscular blocking drug is administered to facilitate endotracheal intubation. Soon after administration, he develops brief generalized muscle fasciculations followed by flaccid paralysis. The blockade cannot be reversed during its initial phase with neostigmine. Which drug was most likely administered?

A 34-year-old man presents with high-grade fever and rapidly progressive pain and swelling of the right knee. He is unable to bear weight. The joint is warm, markedly tender, and held partially flexed. Synovial fluid is turbid with 80,000 leukocytes/mm³ predominantly neutrophils. Gram stain shows gram-positive cocci in clusters. Which organism is the most likely cause?

A 32-year-old man is examined after an assault. He has a 4-cm wound over the scalp produced by a blow from a metal rod. The wound has irregular margins, surrounding abrasion and bruising, and small strands of tissue remain intact across its depth. The investigating officer initially describes the injury as a cut produced by a sharp weapon. Which feature most strongly indicates that this is a laceration rather than an incised wound?

A 12-year-old boy presents with fever and painful swelling of both knees 3 weeks after recovering from a sore throat. The joint pain appears to migrate from one large joint to another. Examination reveals a new apical systolic murmur. His antistreptolysin-O titre is elevated. Which diagnosis is most likely?

A 61-year-old man with chronic kidney disease presents with an acute attack of gout affecting his right great toe. He has a history of peptic ulcer disease complicated by gastrointestinal bleeding. The joint is extremely tender and swollen, and aspiration confirms monosodium urate crystals. Which drug is the most appropriate option for treating the acute attack in this patient?

A 30-year-old construction worker presents after being struck by a sharp metallic object. Examination reveals a clean wound with sharply defined margins and no surrounding bruising. The wound is longer than it is deep and bleeds freely. Which type of mechanical injury is most likely?

A 22-year-old medical student falls on his outstretched right hand while playing football. He develops pain in the anatomical snuffbox. Initial radiographs are inconclusive, but repeat imaging confirms a fracture through the waist of the scaphoid. His surgeon explains that one fragment is particularly vulnerable to avascular necrosis because of the pattern of blood supply. Which portion of the scaphoid is at greatest risk?

A 52-year-old man is scheduled for elective abdominal surgery. During induction of general anesthesia, rocuronium is administered to facilitate endotracheal intubation. At the end of surgery, he has residual skeletal-muscle weakness and inadequate spontaneous ventilation. Which drug can rapidly reverse the neuromuscular blockade produced by rocuronium?

A 40-year-old driver is involved in a frontal collision and sustains a thoracic spinal-cord hemisection. Examination shows ipsilateral loss of motor function and proprioception below the lesion, with contralateral loss of pain and temperature sensation. Which syndrome is most consistent with these findings?

A 37-year-old man presents with sharply demarcated erythematous plaques covered by silvery-white scales over the extensor surfaces of the elbows and knees. Similar lesions are present over the scalp. Removal of scales produces pinpoint bleeding. Which diagnosis is most likely?

A 67-year-old woman presents with persistent lower back pain after lifting a bucket of water. She entered menopause 16 years ago and has never received hormone replacement therapy. She has no history of major trauma. Examination reveals thoracic kyphosis and mild tenderness over the lower thoracic vertebrae. X-ray demonstrates a wedge compression fracture of T12. Serum calcium, phosphate, alkaline phosphatase, and parathyroid hormone levels are within normal limits. DEXA scan shows a T-score of −3.1 at the femoral neck. Which of the following is the most likely underlying disorder?

A 29-year-old man is brought after a firearm injury to the chest. Examination reveals a small round wound with an abrasion collar and an internal beveling defect in the underlying rib. No second wound is identified. The wound described is most consistent with:

A 29-year-old man sustains a closed midshaft tibial fracture in a motorcycle accident. The limb is immobilized appropriately. During follow-up, radiographs demonstrate formation of a soft callus followed several weeks later by mineralized woven bone bridging the fracture fragments. Over subsequent months, the fracture regains its original structural organization. Which process predominates during the final stage of normal fracture healing?

A 26-year-old man is brought to the emergency department after a high-speed motorcycle collision. He complains of severe neck pain and weakness of all four limbs. On arrival, his airway is patent, breathing is adequate, and blood pressure is stable. What is the most appropriate immediate step while assessing this patient?

A 58-year-old man with recurrent attacks of gout is started on allopurinol after his acute attack has completely settled. He has a history of recurrent uric acid renal stones and persistently elevated serum uric acid. What is the principal mechanism of action of allopurinol?

A 45-year-old man sustains a deep laceration of the lower leg. The wound is initially contaminated but is adequately cleaned and sutured. Several days later, fibroblasts proliferate within the wound, new capillaries form, and a soft pink tissue becomes visible at the wound surface. Which tissue is being described?

A 28-year-old man is examined after an assault in which a screwdriver was used. He has a small elliptical wound over the anterior chest measuring 1.5 cm in length. Surgical exploration shows that the wound tract extends 8 cm into the thoracic cavity. How should this injury be classified?

A 42-year-old woman who follows a restrictive diet presents with diffuse bone pain, proximal muscle weakness, and difficulty climbing stairs. She rarely goes outdoors because of her occupation. Examination shows tenderness over the ribs and pelvis but no joint swelling. Laboratory investigations reveal low serum 25-hydroxyvitamin D, low-normal calcium, reduced phosphate, elevated alkaline phosphatase, and elevated parathyroid hormone. Radiographs demonstrate areas of incomplete cortical fractures involving the femoral neck. Which mechanism best explains her skeletal abnormalities?

A 13-year-old girl is diagnosed with acute rheumatic fever after presenting with migratory polyarthritis and carditis following streptococcal pharyngitis. After treatment of the acute episode, her physician recommends long-term intramuscular benzathine penicillin. What is the main purpose of this treatment?

A 38-year-old woman presents with 6 months of symmetrical pain and swelling involving the wrists, metacarpophalangeal joints, and proximal interphalangeal joints. Morning stiffness lasts nearly 2 hours. Anti-CCP antibodies are strongly positive. Despite regular NSAID use, she has persistent synovitis. The rheumatologist plans to initiate methotrexate as a disease-modifying agent. Which additional prescription is commonly given to reduce methotrexate-associated toxicity?

A 29-year-old woman develops an itchy erythematous rash on both wrists 2 days after wearing a new metal bracelet. The eruption corresponds precisely to the areas of skin contact. Which immunological mechanism is most likely responsible?

A 21-year-old woman presents with intensely itchy erythematous patches in the flexural surfaces of both elbows and behind the knees. She has a personal history of allergic rhinitis. The skin is dry, excoriated, and mildly lichenified. Which diagnosis is most likely?

A 34-year-old woman presents with progressive proximal muscle weakness affecting both shoulders and hips. She has difficulty combing her hair and rising from a chair. Serum creatine kinase is markedly elevated. Muscle biopsy shows inflammatory cells surrounding and invading muscle fibres. Which diagnosis is most likely?

A 39-year-old man develops a large burn over the anterior chest. Several months later, the healed area becomes progressively tight and restricts movement of the shoulder. Examination shows shortening and distortion of the scar tissue. Which complication of wound healing is most likely?

A 10-year-old boy presents with gradually worsening pain and swelling around the hip accompanied by low-grade fever and weight loss for 3 months. Imaging demonstrates joint-space narrowing and periarticular osteopenia. Synovial biopsy reveals granulomas containing caseous necrosis. Which diagnosis is most likely?

A 7-month-old infant is brought with irritability and swelling of the left thigh. The mother reports that the child "rolled off the bed." Examination reveals bruises of different appearances over the trunk and extremities. Skeletal survey demonstrates posterior rib fractures, a metaphyseal corner fracture of the femur, and an old healing humeral fracture. The history does not adequately explain the injuries. Which interpretation is most appropriate?

A 25-year-old man reports that he was attacked with a knife. Examination reveals several superficial incised wounds over the ulnar border of the left forearm and palm, in addition to a deeper incised wound over the chest. He states that he raised his hands and forearm to protect himself during the assault. The superficial injuries on the hand and forearm are best classified as:

A 19-year-old student presents with severe nocturnal itching affecting the finger webs, wrists, and waistline. Several family members have developed similar symptoms. Examination reveals small papules and thin serpiginous burrows. Which diagnosis is most likely?

A 61-year-old man develops gradually worsening deep pelvic pain. Imaging demonstrates a destructive lesion of the pelvic bone with areas of calcification described as "rings and arcs." Biopsy reveals malignant cartilage-producing cells within lacunae and significant cellular atypia. Which tumour is the most likely diagnosis?

A 32-year-old woman sustains a burn involving the entire epidermis and extending into the dermis. The affected area is red, blistered, extremely painful, and blanches on pressure. Which classification best describes this burn?

A 30-year-old man is brought after a road-traffic accident. He has loss of motor power in both lower limbs, absent reflexes, and flaccid paralysis immediately after a thoracic spinal-cord injury. There is no evidence of peripheral nerve transection. Which condition best explains this early neurological state?

A 59-year-old man with poorly controlled diabetes mellitus presents with a chronic plantar ulcer beneath the first metatarsal head. The ulcer has been present for 3 months and now probes to bone. He has minimal pain because of peripheral neuropathy. Plain radiographs are equivocal, but inflammatory markers are persistently elevated. Which investigation is most appropriate for detecting early osteomyelitis and defining its anatomical extent?

A 24-year-old woman develops a raised, firm, shiny scar several months after having her ear pierced. The lesion continues to enlarge beyond the original boundaries of the wound. Which abnormality of wound healing is most likely?

A 24-year-old athlete presents with an enlarging pruritic circular lesion on the forearm. Examination shows an erythematous scaly border with central clearing. Potassium hydroxide preparation demonstrates branching septate hyphae. Which diagnosis is most likely?

A 46-year-old woman notices recent enlargement of a pigmented lesion on her upper back. Examination shows an asymmetric lesion with irregular borders, multiple shades of brown and black, and a diameter of 9 mm. Biopsy confirms malignant melanoma. Which histological measurement is most important for determining prognosis?

A 33-year-old man is found dead after a close-range firearm injury to the temple. The skin around the wound shows blackening, burning, and tattooing by unburnt powder particles. Which finding most strongly indicates a close-range firearm discharge?

A 25-year-old man is injured in an explosion inside a closed building. He has no major external trauma but develops severe shortness of breath, chest pain, and hemoptysis. Examination reveals hypoxia and diffuse pulmonary crepitations. Which mechanism is most likely responsible for his pulmonary injury?

A 72-year-old man has been bedridden for 8 weeks following a stroke. Examination reveals marked reduction in the circumference of both thighs, but there is no evidence of peripheral nerve injury. Which cellular adaptation most likely accounts for the reduction in muscle mass?

A 16-year-old boy presents with progressive pain and swelling around the distal femur for 3 months. The pain has become persistent and is occasionally worse at night. Radiography demonstrates a destructive metaphyseal lesion with mixed sclerosis and lucency, cortical disruption, and a sunburst-type periosteal reaction. Biopsy reveals malignant cells producing lace-like osteoid. Which diagnosis is most likely?

A 44-year-old man has active rheumatoid arthritis despite adequate treatment with methotrexate. His rheumatologist is considering adding a tumour necrosis factor-alpha inhibitor. He reports that several years ago he lived with a relative who was treated for pulmonary tuberculosis. He currently has no cough, fever, or weight loss. Which step is most important before initiating anti-TNF therapy?

A 69-year-old woman presents with acute pain and swelling of the right knee. She has no previous history of gout. Radiography demonstrates linear calcification within the articular cartilage. Synovial fluid microscopy reveals rhomboid-shaped crystals with weak positive birefringence. Which diagnosis is most likely?

A 6-year-old girl has recurrent painless swelling of both knees and one ankle for 5 months. She has no fever or preceding gastrointestinal illness. Laboratory investigations show a positive antinuclear antibody, while rheumatoid factor is negative. She is diagnosed with oligoarticular juvenile idiopathic arthritis. Her parents are surprised when the pediatric rheumatologist recommends regular ophthalmological examinations despite the absence of visual symptoms. Which complication is the physician primarily attempting to detect?

A 23-year-old woman presents with inflammatory acne involving the face and upper back. Examination reveals multiple comedones, papules, and pustules without nodules or scarring. She is started on a topical retinoid as part of her treatment. What is the principal therapeutic action of topical retinoids in acne?

A 40-year-old electrician accidentally contacts a high-voltage wire. He is found unconscious and is successfully resuscitated. Examination reveals small electrical burns on the hand and foot. Which immediate life-threatening complication is most important to consider?

A 46-year-old man presents with intermittent purulent discharge from a sinus over the tibia. He sustained an open fracture at the same site 3 years earlier and underwent multiple operations. Radiographs reveal dense irregular bone surrounding a central devitalized fragment, together with cortical thickening and several draining channels. Which term describes the devitalized fragment of bone lying within the infected area?

A 53-year-old man with recurrent renal calculi presents with generalized weakness, constipation, and intermittent bone pain. Serum investigations reveal calcium 11.8 mg/dL, phosphate 2.1 mg/dL, and an elevated parathyroid hormone concentration. Radiographs of the hands demonstrate subperiosteal bone resorption along the radial aspects of the middle phalanges. Which cellular effect of parathyroid hormone is primarily responsible for his skeletal changes?

An 8-year-old boy presents with high-grade fever, severe pain around the distal femur, and refusal to bear weight. There is localized warmth and tenderness without an obvious wound. His leukocyte count and C-reactive protein are elevated. MRI demonstrates marrow edema and a metaphyseal collection in the distal femur. Blood cultures are obtained before starting antimicrobial therapy. Which organism is the most likely cause?

A 72-year-old man presents with a scaly erythematous lesion on the dorsum of his hand that has gradually become thickened and ulcerated. Biopsy demonstrates malignant squamous cells forming keratin pearls. Which diagnosis is most likely?

A 5-year-old boy is brought because of frequent falls, difficulty running, and inability to rise easily from the floor. When standing, he uses his hands to push on his knees and thighs. Examination demonstrates calf enlargement. Serum creatine kinase is markedly elevated. Which diagnosis is most likely?

A 68-year-old farmer with long-term sun exposure presents with a slowly enlarging pearly papule on the side of his nose. The lesion has surface telangiectasia and a rolled border. It occasionally ulcerates but has not produced regional lymph-node enlargement. Which diagnosis is most likely?

A 9-year-old boy presents with multiple superficial vesicles and pustules around the nose and mouth. Several lesions have ruptured, leaving honey-colored crusts. He is otherwise well. Which organism is a common cause of this condition?

A 52-year-old man presents with sudden severe pain, redness, and swelling of the first metatarsophalangeal joint that began during the night. He attended a wedding the previous evening where he consumed a large meal rich in red meat. Joint aspiration reveals needle-shaped crystals showing strong negative birefringence under polarized light. Which crystal is responsible for this attack?

A 7-year-old boy presents with a rapidly enlarging soft-tissue mass arising from the muscles of the thigh. Histological examination reveals primitive malignant cells showing skeletal-muscle differentiation with cross-striations in some cells. Which tumour is most likely?

A 64-year-old overweight woman presents with gradually progressive bilateral knee pain for 5 years. Pain worsens with walking and improves with rest. Morning stiffness lasts approximately 10 minutes. Examination reveals crepitus and bony enlargement without marked warmth. Radiographs show asymmetrical joint-space narrowing, subchondral sclerosis, and marginal osteophytes. Which pathological process best explains her condition?