BONUS WEEKEND QUESTION #3

TOPIC: MSK Pathology.

Nov 23, 2024

Today’s bonus weekend question will test your knowledge of a commonly tested, high-yield musculoskeletal pathology.

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A 45-year-old man presents with a three-month history of worsening right hip pain that is exacerbated by weight-bearing activities and relieved by rest. He has a history of chronic alcohol use and reports no history of trauma. On physical examination, there is limited range of motion of the hip, particularly with internal rotation and abduction. Laboratory studies are unremarkable. Plain radiographs of the hip show a crescent sign in the femoral head. Which of the following best describes the underlying pathophysiology responsible for this patient's condition?

A) Increased subchondral blood flow leading to cartilage overgrowth
B) Repetitive microtrauma to the femoral head disrupting blood supply
C) Inherited genetic mutation affecting collagen synthesis in the femoral head
D) Ischemic injury to the femoral head resulting in osteocyte death and collapse of the bone
E) Increased intramedullary pressure within the femoral head causing vascular compression

Detailed Breakdown of Answers + Correct Answer Below ⏬

ANSWER + QUESTION BREAKDOWN

It’s important to adopt the correct MENTAL MODEL when answering USMLE questions; it saves time and increases accuracy. The mental model outlined below is a foundational component of our 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 best describes the underlying pathophysiology responsible for this patient's condition?”

Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Diagnose the condition; 2nd: Identify the correct underlying pathophysiology of the condition.

Step 3: Read the vignette carefully and ask yourself: “What’s the underlying cause of this condition?”

Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.

GENERAL ANALYSIS

This vignette describes a 45-year-old man with a 3-month history of worsening right hip pain, exacerbated by weight-bearing activities and relieved by rest. Physical exam and imaging findings strongly suggest avascular necrosis of the femoral head.

ANSWER CHOICES:

CHOICE A: Increased subchondral blood flow leading to cartilage overgrowth

Explanation: This option suggests an increase in blood flow, leading to overgrowth of cartilage. However, osteonecrosis is caused by decreased blood flow, not increased blood flow. Osteonecrosis is due to ischemia (lack of blood supply), not increased blood flow.

CHOICE B: Repetitive microtrauma to the femoral head disrupting blood supply

Explanation: Repetitive microtrauma can disrupt blood supply and lead to osteonecrosis. However, this patient does not have a history of trauma or repetitive stress injuries. While repetitive microtrauma can contribute to osteonecrosis in some cases (e.g., in athletes), this patient’s history of chronic alcohol use is a more relevant factor. Alcohol use can cause fatty deposits in blood vessels, leading to ischemia.

CHOICE C: CT scan of the abdomen and pelvis with contrast

Explanation: Genetic mutations affecting collagen synthesis are seen in conditions like osteogenesis imperfecta but are unrelated to osteonecrosis. .

CHOICE D: Ischemic injury to the femoral head resulting in osteocyte death and collapse of the bone

Explanation: Osteonecrosis occurs when there is an interruption in the blood supply to the femoral head, leading to ischemic injury. This results in the death of osteocytes and eventual collapse of the bone, as seen with the crescent sign on radiographs.

CHOICE E: Increased intramedullary pressure within the femoral head causing vascular compression

Explanation: Increased intramedullary pressure can compress blood vessels and reduce blood flow. While this may play a role in certain cases (e.g., steroid-induced osteonecrosis), it is not the primary mechanism in most cases. Although increased intramedullary pressure can contribute to vascular compression, ischemic injury from impaired blood flow (as described in option D) better explains this patient’s condition.

FINAL VERDICT…

Based on his symptoms and imaging findings (crescent sign), the most likely diagnosis for this patient is osteonecrosis (avascular necrosis) of the femoral head. The pathophysiology involves an interruption in blood supply to the femoral head, leading to ischemia, death of bone cells (osteocytes), and eventual collapse of the bone structure. Chronic alcohol use is a well-known risk factor for osteonecrosis because it can lead to fatty deposits that block small blood vessels supplying the bone.

Key Concepts:

Osteonecrosis (avascular necrosis) occurs when the bone’s blood supply is interrupted. This leads to ischemia, osteocyte death, and eventual collapse of the bone.
The crescent sign on radiographs is a subchondral radiolucent line that indicates subchondral fracture due to underlying bone collapse.
Risk factors for osteonecrosis include chronic alcohol use, corticosteroid use, trauma, and certain medical conditions.

CORRECT ANSWER: D) Ischemic injury to the femoral head resulting in osteocyte death and collapse of the bone