Question 1: Rheumatoid Arthritis
Stem: A 55-year-old woman presents with symmetrical, bilateral pain and swelling in her hands, worst in the morning. A radiograph of her hands reveals periarticular osteopenia, symmetric joint space narrowing, and marginal erosions at the 2nd and 3rd metacarpophalangeal (MCP) joints.
Question: What is the most likely diagnosis?
(A) Rheumatoid Arthritis (RA) (B) Psoriatic Arthritis (C) Osteoarthritis (D) Gout (E) CPPD (Pseudogout)
Correct Answer: (A) Rheumatoid Arthritis (RA).
Explanation:
- Why (A) is correct: This is the classic triad of RA. It is a symmetric, erosive polyarthropathy. The key features are periarticular osteopenia (loss of bone density around the joint), symmetric joint space loss, and marginal erosions (which start at the “bare areas” of the joint). The MCPs are a classic location.
- Why (B) is wrong: Psoriatic arthritis is typically asymmetric, affects the DIP joints, and is associated with new bone formation (periostitis), not osteopenia.
- Why (C) is wrong: Osteoarthritis is asymmetric, non-erosive (in this manner), and affects different joints (DIPs, PIPs, 1st CMC). It is characterized by osteophytes and subchondral sclerosis, not osteopenia.
- Why (D) is wrong: Gout causes “punched-out” lytic lesions with sclerotic borders and overhanging edges, and it preserves bone density.
- Why (E) is wrong: CPPD causes chondrocalcinosis (cartilage calcification) and has a different pattern of arthropathy, classically at the MCPs and wrist.
Key Points: Rheumatoid Arthritis
- Pathology: A systemic, inflammatory, autoimmune disease attacking the synovium.
- Distribution: Symmetric polyarthropathy. Classically involves MCPs, PIPs, and wrists. Spares the DIPs.
- Radiographic Hallmarks:
- Soft tissue swelling (early).
- Periarticular osteopenia (early).
- Symmetric joint space narrowing.
- Marginal erosions (late).
- Late Deformities: Ulnar deviation, swan neck, and boutonnière deformities.
Question 2: Psoriatic Arthritis
Stem: A 40-year-old man with a history of a chronic skin rash on his scalp and elbows presents with pain in his right 3rd toe. An X-ray of the foot shows severe, destructive changes of the 3rd distal interphalangeal (DIP) joint. There is erosion of the head of the middle phalanx and the base of the distal phalanx, creating a “pencil-in-cup” appearance.
Question: What is the most likely diagnosis?
(A) Rheumatoid Arthritis (B) Psoriatic Arthritis (C) Gout (D) Septic Arthritis (E) Osteoarthritis
Correct Answer: (B) Psoriatic Arthritis.
Explanation:
- Why (B) is correct: The “pencil-in-cup” deformity is pathognomonic for psoriatic arthritis. It is a seronegative spondyloarthropathy (like ankylosing spondylitis) and is characterized by a “dirty” combination of erosion and new bone formation (periostitis). It classically affects the DIP joints.
- Why (A) is wrong: RA spares the DIP joints and causes clean erosions without new bone formation.
- Why (C) is wrong: Gout causes “rat-bite” erosions, not the “pencil-in-cup” deformity.
- Why (D) is wrong: Septic arthritis is an acute, monoarticular infection that rapidly destroys the joint, but it doesn’t create this specific deformity.
- Why (E) is wrong: Osteoarthritis causes osteophytes (new bone) but not the severe central erosions.
Key Points: Psoriatic Arthritis
- Pathology: A seronegative inflammatory arthritis associated with psoriasis.
- Distribution: Often asymmetric. Classically involves the DIP joints of the hands and feet.
- Radiographic Hallmarks:
- “Pencil-in-cup” deformity (pathognomonic).
- Erosion + New Bone Formation (periostitis) – the “dirty” arthritis.
- Acro-osteolysis (resorption of the distal tufts).
- “Sausage digit” (dactylitis) due to soft tissue swelling.
Question 3: Gout
Stem: A 55-year-old man with a history of renal failure presents with an acutely painful and swollen 1st metatarsophalangeal (MTP) joint. A radiograph shows a large, “punched-out” lytic lesion with a sclerotic border and overhanging edges at the medial aspect of the 1st metatarsal head. There is an associated dense soft tissue lump.
Question: This appearance is classic for:
(A) Rheumatoid Arthritis (B) Osteoarthritis (C) Tophaceous Gout (D) CPPD (Pseudogout) (E) Septic Arthritis
Correct Answer: (C) Tophaceous Gout.
Explanation:
- Why (C) is correct: This is the classic “rat-bite” erosion of chronic gout. The key features are: 1) Punched-out erosions with sclerotic borders and overhanging edges (the “Martel sign”); 2) Location away from the joint margin (juxta-articular); 3) Preservation of bone density and joint space until late-stage; 4) Associated dense soft tissue tophi.
- Why (A) is wrong: RA causes marginal erosions and osteopenia.
- Why (B) is wrong: OA causes osteophytes and lacks erosions.
- Why (D) is wrong: CPPD causes cartilage calcification, not erosions.
- Why (E) is wrong: Septic arthritis is an acute destructive process and does not form chronic tophi or sclerotic-bordered erosions.
Key Points: Gout
- Pathology: A metabolic disorder; deposition of monosodium urate (MSU) crystals.
- Location: Classically the 1st MTP joint (podagra), but also hands, elbows (olecranon bursa), and knees.
- Radiographic Hallmarks (Chronic):
- “Rat-bite” erosions: Punched-out, juxta-articular, sclerotic borders, overhanging edges.
- Dense soft tissue tophi (which may calcify).
- Preservation of joint space and bone density.
Question 4: CPPD (Pseudogout)
Stem: An 80-year-old woman presents with acute knee pain. A radiograph of her knee shows a thin, linear, white line of calcification within the joint space, parallel to the articular surface of the femoral condyles and tibial plateau. There is also calcification in the menisci.
Question: This finding is diagnostic of:
(A) Chondrocalcinosis (CPPD) (B) Gout (C) Advanced Osteoarthritis (D) Osteochondritis Dissecans (E) Hydroxyapatite Deposition Disease (HADD)
Correct Answer: (A) Chondrocalcinosis (CPPD).
Explanation:
- Why (A) is correct: The finding described is chondrocalcinosis, which is the calcification of hyaline cartilage and fibrocartilage (like the menisci). This is the pathognomonic finding of Calcium Pyrophosphate Dihydrate (CPPD) deposition disease (pseudogout).
- Why (B) is wrong: Gout causes soft tissue tophi, not cartilage calcification.
- Why (C) is wrong: Osteoarthritis involves osteophytes and sclerosis, but not linear calcification of the cartilage.
- Why (D) is wrong: This is a focal defect in the articular surface, not a diffuse calcification.
- Why (E) is wrong: HADD causes calcification of tendons and bursae (e.g., supraspinatus), not articular cartilage.
Key Points: CPPD (Pseudogout)
- Definition: Deposition of calcium pyrophosphate crystals.
- Hallmark Sign: Chondrocalcinosis.
- Classic Locations for Chondrocalcinosis:
- Knee: Menisci and hyaline cartilage (linear).
- Wrist: Triangular Fibrocartilage Complex (TFCC).
- Pubic Symphysis.
- Arthropathy Pattern: Can mimic OA, but in atypical locations (e.g., radiocarpal joint, MCP joints).
Question 5: Septic Arthritis
Stem: A 5-year-old boy presents with a 2-day history of high fever, refusal to bear weight, and exquisite pain on any movement of his left hip. An ultrasound demonstrates a large, complex joint effusion with thickened, hyperaemic synovium.
Question: What is the most important, time-sensitive diagnosis to exclude?
(A) Transient Synovitis (Irritable Hip) (B) Perthes’ Disease (C) Septic Arthritis (D) Juvenile Idiopathic Arthritis (E) Slipped Upper Femoral Epiphysis (SUFE)
Correct Answer: (C) Septic Arthritis.
Explanation:
- Why (C) is correct: The clinical triad of fever, non-weight-bearing, and an acute inflammatory picture (high ESR/CRP) in a child points to septic arthritis. This is a surgical emergency. The ultrasound findings of a complex effusion and hyperaemia support this.
- Why (A) is wrong: Transient synovitis is the most common cause of a painful hip in a child, but it is a diagnosis of exclusion. The child is not typically febrile or systemically unwell, and the effusion is simple.
- Why (B) & (E) are wrong: Perthes’ and SUFE are chronic, mechanical problems, not acute, febrile illnesses.
- Why (D) is wrong: JIA is a chronic inflammatory process and would not present this acutely.
Key Points: Septic Arthritis
- Definition: Bacterial infection of a joint space.
- Clinical: Surgical emergency! Presents with acute pain, fever, and inability to bear weight.
- Pathogen: Staph. aureus is most common.
- Imaging:
- Ultrasound: The primary modality. Shows a joint effusion (often complex/echogenic).
- X-ray: May be normal or show joint space widening (early), then rapid cartilage/bone destruction (late).
- Note: In a child, must be distinguished from transient synovitis. Fever and high inflammatory markers point to infection.
Question 6: Paget’s Disease
Stem: A 72-year-old man presents with an elevated alkaline phosphatase. A radiograph of his pelvis shows thickening of the iliopectineal line (pelvic brim sign) and coarsening of the trabecular pattern in the right hemipelvis. A CT of his skull shows a “cotton wool” appearance.
Question: What is the most likely diagnosis?
(A) Paget’s Disease of Bone (B) Sclerotic Metastases (e.g., from prostate) (C) Fibrous Dysplasia (D) Myelofibrosis (E) Renal Osteodystrophy
Correct Answer: (A) Paget’s Disease of Bone.
Explanation:
- Why (A) is correct: This is the classic multi-site presentation. The key findings are bone expansion, cortical thickening (e.g., “pelvic brim sign”), and coarsening of the trabeculae. The “cotton wool” appearance of the skull is a classic sign of the mixed lytic/sclerotic phase.
- Why (B) is wrong: Sclerotic metastases are dense but do not expand the bone, coarsen the trabeculae, or thicken the cortex in this manner.
- Why (C) is wrong: Fibrous dysplasia is a “ground-glass” lesion of younger patients.
- Why (D) is wrong: Myelofibrosis causes diffuse bone sclerosis but not the focal, expansile changes of Paget’s.
- Why (E) is wrong: This has a different appearance (e.g., “rugger-jersey spine”).
Key Points: Paget’s Disease of Bone
- Pathology: A chronic disorder of accelerated, abnormal bone remodelling (disorganised osteoclast/osteoblast activity).
- Phases: 1. Lytic (e.g., “osteoporosis circumscripta” in skull), 2. Mixed, 3. Sclerotic (“ivory” vertebra).
- Key Imaging Signs:
- Bone Expansion.
- Cortical Thickening (“pelvic brim sign”).
- Coarsening of Trabeculae.
- Skull Signs: “Osteoporosis circumscripta” (lytic), “Cotton wool” (mixed).
- Complication: Malignant transformation to osteosarcoma (<1%).
Question 7: Hyperparathyroidism
Stem: A 50-year-old woman with chronic renal failure presents with bone pain. A radiograph of her hand is performed, which reveals acro-osteolysis (resorption of the distal tufts) and subperiosteal bone resorption, particularly along the radial aspect of the 2nd and 3rd middle phalanges.
Question: This finding is pathognomonic for:
(A) Hyperparathyroidism (B) Psoriatic Arthritis (C) Scleroderma (D) Gout (E) Rheumatoid Arthritis
Correct Answer: (A) Hyperparathyroidism.
Explanation:
- Why (A) is correct: Subperiosteal bone resorption is the pathognomonic sign of hyperparathyroidism (HPT). The earliest and most classic location for this finding is the radial aspect of the middle phalanges. The acro-osteolysis is also a feature.
- Why (B) is wrong: Psoriatic arthritis causes acro-osteolysis but is associated with new bone formation (periostitis), not subperiosteal resorption.
- Why (C) is wrong: Scleroderma causes acro-osteolysis and soft tissue calcification, but not subperiosteal resorption.
- Why (D) & (E) are wrong: Gout and RA cause erosions, which are focal defects, not the “pencil-thinning” resorption seen under the periosteum.
Key Points: Hyperparathyroidism (HPT)
- Pathology: Overactive parathyroid glands (can be Primary, Secondary [e.g., renal failure], or Tertiary).
- Pathognomonic Sign: Subperiosteal bone resorption (radial aspect of middle phalanges).
- Other Signs:
- Brown Tumours: Expansile, lytic lesions (osteoclastomas).
- Acro-osteolysis.
- “Rugger-Jersey Spine” (sclerotic endplates).
- “Salt-and-Pepper” Skull.
Question 8: Rickets
Stem: A 2-year-old child presents with bowed legs and delayed motor milestones. A radiograph of the knee shows that the metaphyses are widened, frayed, and “cupped”. The overall bone density is poor.
Question: What is the most likely diagnosis?
(A) Rickets (Osteomalacia) (B) Scurvy (C) Blount’s Disease (D) Non-Accidental Injury (NAI) (E) Achondroplasia
Correct Answer: (A) Rickets (Osteomalacia).
Explanation:
- Why (A) is correct: This is the classic description of rickets (the childhood form of osteomalacia), caused by Vitamin D deficiency. The growth plates (physes) are unable to mineralize, leading to a build-up of unmineralized osteoid. This appears as widening of the physis and “fraying” and “cupping” of the metaphysis.
- Why (B) is wrong: Scurvy (Vitamin C deficiency) has different signs, such as a “white line of Frankel” (dense metaphyseal band), cortical thinning, and a “Wimberger ring” around the epiphysis.
- Why (C) is wrong: Blount’s disease is a specific growth disturbance of the medial proximal tibial physis, causing bowing (varus), not a generalized metabolic picture.
- Why (D) is wrong: NAI would show fractures (e.g., metaphyseal corner fractures), not physeal fraying.
- Why (E) is wrong: Achondroplasia is a genetic dwarfism with short, thick long bones.
Key Points: Rickets
- Definition: Defective mineralization of bone and growth plate osteoid in children, usually due to Vitamin D deficiency.
- Hallmarks: Widening and irregularity of the growth plates.
- Classic Signs:
- “Fraying” of the metaphysis.
- “Cupping” of the metaphysis.
- Bowing of the long bones.
- “Rachitic Rosary” (expansion of the costochondral junctions).
Question 9: Renal Osteodystrophy
Stem: A 58-year-old man on long-term haemodialysis has a lateral spine X-ray. The vertebral bodies show increased density at the superior and inferior endplates, with a relative lucency in the centre.
Question: This appearance is known as the:
(A) “Rugger-Jersey Spine” (B) “Ivory Vertebra” (C) “Picture Frame” Vertebra (D) “Bamboo Spine” (E) “Bone-within-bone” appearance
Correct Answer: (A) “Rugger-Jersey Spine”.
Explanation:
- Why (A) is correct: The “rugger-jersey spine” (named after the alternating stripes on a rugby shirt) is the classic sign of renal osteodystrophy. This is a complex metabolic bone disease caused by chronic renal failure, which results in secondary hyperparathyroidism. The dense endplates are due to osteosclerosis.
- Why (B) is wrong: An “ivory vertebra” is a single, uniformly dense vertebra, classic for lymphoma or Paget’s disease.
- Why (C) is wrong: A “picture frame” vertebra (thickened cortex on all 4 sides) is a sign of Paget’s disease.
- Why (D) is wrong: A “bamboo spine” is from the syndesmophytes of ankylosing spondylitis.
- Why (E) is wrong: This is seen in osteopetrosis.
Key Points: Renal Osteodystrophy
- Definition: A complex bone disease in chronic renal failure.
- Mechanism: It is a combination of secondary hyperparathyroidism and osteomalacia.
- Key Signs:
- “Rugger-Jersey Spine” (sclerotic endplates).
- Subperiosteal resorption (from the HPT component).
- Soft tissue (“tumoral”) calcification.
Question 10: Tumour Location (Epiphysis)
Stem: A 16-year-old boy presents with knee pain. An MRI demonstrates a well-defined, lytic lesion with a thin sclerotic rim, located entirely within the epiphysis of the distal femur.
Question: Given the patient’s age and the purely epiphyseal location, what is the most likely diagnosis?
(A) Chondroblastoma (B) Giant Cell Tumour (GCT) (C) Non-Ossifying Fibroma (NOF) (D) Aneurysmal Bone Cyst (ABC) (E) Osteosarcoma
Correct Answer: (A) Chondroblastoma.
Explanation:
- Why (A) is correct: Chondroblastoma is the classic “C” of the epiphyseal lesion differential. It is a benign but locally aggressive tumour that occurs in skeletally immature patients (age 10-20, growth plates open).
- Why (B) is wrong: A GCT is also epiphyseal, but it occurs in skeletally MATURE patients (age 20-40, growth plates closed).
- Why (C) & (D) are wrong: NOFs and ABCs are classically metaphyseal lesions.
- Why (E) is wrong: Osteosarcoma is classically metaphyseal.
Key Points: Epiphyseal Lesion Differential
- A helpful mnemonic is F-A-C-E-S:
- F – Infection (Brodie’s abscess)
- A – Aneurysmal Bone Cyst (ABC) – Rarely, can be epiphyseal
- C – Chondroblastoma (in skeletally immature)
- E – Eosinophilic Granuloma / Enchondroma
- S – Giant Cell Tumour (GCT) (in skeletally mature)
Question 11: Tumour Location (Diaphysis)
Stem: A 15-year-old girl presents with leg pain and fever. An X-ray shows a permeative, lytic lesion in the diaphysis (shaft) of the femur, with a lamellated “onion-skin” periosteal reaction.
Question: Which of the following tumours is the most likely diagnosis given this classic location and appearance?
(A) Ewing’s Sarcoma (B) Osteosarcoma (C) Chondrosarcoma (D) Giant Cell Tumour (GCT) (E) Osteoid Osteoma
Correct Answer: (A) Ewing’s Sarcoma.
Explanation:
- Why (A) is correct: Ewing’s sarcoma is the classic diaphyseal malignancy of childhood. Its appearance (permeative, “onion-skin”) and clinical presentation (mimicking infection with fever) are characteristic.
- Why (B) is wrong: Osteosarcoma is classically metaphyseal.
- Why (C) is wrong: Chondrosarcoma is a tumour of older adults.
- Why (D) is wrong: GCT is epiphyseal.
- Why (E) is wrong: Osteoid osteoma is typically in the cortex of the diaphysis or metaphysis, but it is a small nidus with sclerosis, not a large, permeative lesion.
Key Points: Diaphyseal Lesion Differential
- A helpful mnemonic is F-E-M-A-L-E-S:
- F – Fibrous Dysplasia
- E – Ewing’s Sarcoma, Eosinophilic Granuloma
- M – Metastases, Myeloma
- A – Adamantinoma
- L – Lymphoma
- E – Enchondroma
- S – Simple Bone Cyst
Question 12: Tumour Malignancy (Zone of Transition)
Stem: A radiologist is describing a lytic bone lesion. They state that the lesion has a “wide zone of transition” with the normal bone.
Question: This descriptive term implies that the lesion is most likely:
(A) Malignant (B) Benign (C) Infectious (D) Traumatic (E) Metabolic
Correct Answer: (A) Malignant.
Explanation:
- Why (A) is correct: The zone of transition is the single most important predictor of a lesion’s aggressiveness. A wide zone of transition (also called “ill-defined” or “imperceptible” border) means the tumour is growing rapidly and infiltrating, leaving no time for the host bone to react. This is a hallmark of malignancy (e.g., sarcoma, metastasis).
- Why (B) is wrong: A benign lesion is slow-growing, allowing the host bone to form a narrow zone of transition (a sharp, well-defined, often sclerotic border).
- Why (C) is wrong: An acute infection (osteomyelitis) can also have a wide zone of transition, but “malignant” is the best general answer, as this is the primary radiological distinction.
- Why (D) & (E) are wrong: These are not primary descriptors.
Key Points: Benign vs. Malignant Features
- Benign (Slow-growing):
- Narrow zone of transition.
- Well-defined, sclerotic border.
- No soft-tissue mass.
- Malignant (Aggressive):
- Wide zone of transition (ill-defined).
- Permeative or “moth-eaten” destruction.
- Soft-tissue mass.
- Aggressive periosteal reaction (sunburst, Codman’s).
Question 13: Myositis Ossificans
Stem: A 20-year-old football player sustained a deep thigh bruise (quadriceps haematoma) 6 weeks ago. He now has a hard, palpable lump. An X-ray shows a well-circumscribed, ovoid mass in the soft tissues of the thigh. The mass has a dense, mature, calcified rim and a lucid, non-calcified centre.
Question: What is the most likely diagnosis?
(A) Myositis Ossificans (B) Parosteal Osteosarcoma (C) Synovial Sarcoma (D) Tophaceous Gout (E) Rhabdomyolysis
Correct Answer: (A) Myositis Ossificans.
Explanation:
- Why (A) is correct: This is the classic presentation and appearance. Myositis ossificans is a benign, reactive process (not a tumour) of bone formation within muscle, typically after trauma. The key imaging feature is the zonal pattern of maturation: it calcifies from the outside in, resulting in a dense peripheral rim and a lucent, immature centre.
- Why (B) is wrong: A parosteal osteosarcoma is a surface malignancy. It has the opposite pattern: it is densest in the centre and less mature at the periphery.
- Why (C) is wrong: A synovial sarcoma is a soft-tissue mass that may have calcifications, but it does not have this organized, zonal-maturation pattern.
- Why (D) is wrong: Gouty tophi are dense, but not in this pattern.
- Why (E) is wrong: Rhabdomyolysis is an acute muscle breakdown and has no calcification.
Key Points: Myositis Ossificans
- Definition: A benign, reactive proliferation of heterotopic bone in soft tissue, usually post-traumatic.
- Key Imaging Sign: Peripheral, mature, dense rim of calcification with a central lucent (immature) zone.
- Note: This “zonal” pattern is the opposite of a parosteal osteosarcoma.
Question 14: Lytic Metastasis
Stem: A 65-year-old smoker presents with back pain. An AP radiograph of the thoracic spine shows that the left pedicle of T7 is absent.
Question: This finding is known as the “winking owl sign” and is highly suggestive of:
(A) Osteolytic Metastasis (B) Spondylolysis (C) Congenital Pedicle Agenesis (D) Multiple Myeloma (E) Paget’s Disease
Correct Answer: (A) Osteolytic Metastasis.
Explanation:
- Why (A) is correct: The “winking owl sign” refers to the appearance of a vertebra on an AP view where one pedicle (the “eye”) is gone. The pedicles are the first structure to be destroyed by osteolytic (destructive) metastases because they have a rich blood supply.
- Why (B) is wrong: Spondylolysis is a fracture of the pars interarticularis (seen on oblique views), not a destruction of the pedicle.
- Why (C) is wrong: Congenital absence is extremely rare, and the contralateral pedicle would be hypertrophied and sclerotic as a compensatory change.
- Why (D) is wrong: Multiple myeloma classically spares the pedicles, even when the vertebral body is heavily involved.
- Why (E) is wrong: Paget’s disease expands and thickens the bone, it does not destroy a pedicle.
Key Points: Metastatic Spine Disease
- Lytic Metastases (Bone-destroying):
- Classic primaries: Lung, Renal, Thyroid, Breast.
- “Winking Owl Sign”: Destruction of a pedicle.
- Sclerotic Metastases (Bone-forming):
- Classic primary: Prostate.
- Key Differentiator: Metastases (both types) spare the disc space, which distinguishes them from pyogenic infection.
Question 15: Benign Bone Tumour (Simple Bone Cyst)
Stem: A 12-year-old boy has an X-ray after a fall, revealing a pathologic fracture through the proximal humerus. The lesion is a central, lytic, and well-defined lesion in the metaphysis. A small fragment of bone is seen to have “dropped” to the bottom of the lytic lesion.
Question: This “dropped fragment” is known as the “fallen fragment sign” and is pathognomonic for:
(A) Simple Bone Cyst (Unicameral Bone Cyst) (B) Aneurysmal Bone Cyst (ABC) (C) Non-Ossifying Fibroma (NOF) (D) Metastasis (E) Osteosarcoma
Correct Answer: (A) Simple Bone Cyst (Unicameral Bone Cyst).
Explanation:
- Why (A) is correct: A simple bone cyst is a fluid-filled cavity. When a pathologic fracture occurs, a small piece of the cortex can “fall” through the fluid to the most dependent part of the cyst. This “fallen fragment sign” is pathognomonic, as it proves the lesion is fluid-filled and not solid.
- Why (B) is wrong: An ABC is blood-filled but is multicystic and septated, so a fragment would not “fall” to the bottom. It has fluid-fluid levels.
- Why (C) is wrong: An NOF is a solid fibrous lesion.
- Why (D) & (E) are wrong: Metastases and osteosarcomas are solid malignant tumours.
Key Points: Simple Bone Cyst (SBC)
- Definition: A benign, fluid-filled, “do-not-touch” lesion.
- Demographic: Children (5-15 years).
- Location: Central, metaphyseal. Classically in the proximal humerus or proximal femur.
- Appearance: Well-defined, lytic, slightly expansile.
- Pathognomonic Sign: “Fallen Fragment Sign” (if fractured).
