USMLE PRACTICE QUESTION #93
TOPIC: MSK Oncology
Today’s question will test your understanding of an important, highly tested musculoskeletal malignancy.
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A 10-year-old boy is brought to the pediatrician by his parents, concerned about persistent pain in his left leg. The pain has been present for the past three months and has gradually worsened. The patient denies any recent injuries, but his parents note that he has been limping recently. The left leg appears swollen around the knee on physical examination, with mild tenderness on palpation. A leg radiograph reveals an expansile, lytic lesion with a radiating, spiculated periosteal reaction and a lifted periosteum at the distal metaphysis of the femur. Based on the patient's presentation and radiographic findings, which of the following is the most likely diagnosis?
A) Osteosarcoma
B) Ewing sarcoma
C) Chondrosarcoma
D) Giant cell tumor of bone
E) Osteochondroma
F) Simple bone cyst
G) Aneurysmal bone cyst
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
The MENTAL MODEL used to answer this question comes from our detailed test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:
Step 1. Read the last line to get to the heart of the question: “Which of the following is the most likely diagnosis?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 1st order. 1st: Diagnose the condition.
Step 3: Read the vignette carefully and ask yourself: “This patient’s findings are most likely due to ________________?”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
The 10-year-old boy presents with persistent leg pain, limping, and swelling around the knee, with radiographic findings of an expansile, lytic lesion, radiating, spiculated periosteal reaction, and a lifted periosteum at the distal metaphysis of the femur. These features are highly suggestive of the most common primary bone malignancy in children and adolescents.
ANSWER CHOICES:
CHOICE A: Osteosarcoma
Explanation: Osteosarcoma is the most common primary malignant bone tumor in children and adolescents. It typically arises in the metaphysis of long bones, particularly around the knee (distal femur or proximal tibia), which matches this patient's presentation.
CHOICE B: Ewing sarcoma
Explanation: Ewing sarcoma is another malignant bone tumor that commonly affects children and adolescents. It typically arises in the diaphysis of long bones or in flat bones like the pelvis. While Ewing sarcoma can present with a periosteal reaction, it more commonly shows an "onion-skin" appearance on radiographs. The location (metaphysis) and radiographic findings in this case are more characteristic of osteosarcoma.
CHOICE C: Chondrosarcoma
Explanation: Chondrosarcoma is a malignant tumor that produces a cartilage matrix. It is more common in adults over 40 and typically affects the pelvis, ribs, or proximal long bones. Chondrosarcoma is rare in children, and its radiographic appearance often includes calcifications described as "rings-and-arcs," which are not mentioned in this case.
CHOICE D: Giant cell tumor of bone
Explanation: Giant cell tumors are benign but locally aggressive tumors that typically occur after skeletal maturity (in adults). They usually arise in the epiphysis of long bones. This patient is skeletally immature, and the lesion is located in the metaphysis, not the epiphysis, making this diagnosis unlikely.
CHOICE E: Osteochondroma
Explanation: Osteochondroma is a benign bone tumor that presents as an exophytic growth (a bony outgrowth capped by cartilage) from the metaphysis of long bones. Osteochondromas are benign and do not present with aggressive features like a spiculated periosteal reaction or Codman’s triangle.
CHOICE F: Simple bone cyst
Explanation: A simple bone cyst is a benign fluid-filled cavity typically occurring in children and adolescents. It often affects the proximal humerus or femur. Simple bone cysts are not associated with aggressive periosteal reactions or cortical destruction seen on this patient's radiograph.
CHOICE G: Aneurysmal bone cyst
Explanation: Aneurysmal bone cysts are benign but expansile lesions filled with blood-filled spaces. They can cause cortical thinning but do not typically cause aggressive periosteal reactions. While these can cause swelling and pain, they do not present with the aggressive features seen on this patient's radiograph.
FINAL VERDICT…
Osteosarcoma is the most common primary malignant bone tumor in children and adolescents. It typically arises in the metaphysis of long bones, particularly around the distal femur or proximal tibia. The radiographic findings described—lytic lesion, spiculated periosteal reaction (aka "sunburst" pattern), and Codman’s triangle (lifting of the periosteum)—are classic signs of osteosarcoma. Osteosarcoma often presents with localized pain, swelling, and sometimes a palpable mass. The pain may worsen at night or with activity.
CORRECT ANSWER: A) Osteosarcoma
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That’s it for question 93!
See ya tomorrow 👋