A 22-year-old female presents to the emergency department with 18 hours of periumbilical pain that has now localised to the right iliac fossa. She has fever (38.1°C), anorexia, and vomiting. On examination, she has tenderness at McBurney's point with positive Rovsing's sign. Laboratory investigations show leucocytosis (14,200/μL, 86% neutrophils) and elevated CRP (64 mg/L). Graded-compression ultrasound of the right lower quadrant is performed. The structure marked **B** in the diagram shows an outer-to-outer diameter of 9 mm. Which of the following best describes the diagnostic significance of this measurement in acute appendicitis?
A. Outer-to-outer diameter <6 mm excludes appendicitis and indicates normal appendix
B. Outer-to-outer diameter ≥6 mm is the primary ultrasound criterion for diagnosis of acute appendicitis and indicates transmural inflammation
C. Outer-to-outer diameter ≥9 mm is pathognomonic for perforated appendicitis with abscess formation
D. Outer-to-outer diameter measurement is unreliable in females due to hormonal variation and should not be used for diagnosis
Explanation
Why "Outer-to-outer diameter ≥6 mm is the primary ultrasound criterion for diagnosis of acute appendicitis and indicates transmural inflammation" is right
In graded-compression ultrasound, an outer-to-outer (maximal) appendiceal diameter ≥6 mm is the key diagnostic criterion for acute appendicitis. The patient's appendix measured 9 mm, which exceeds this threshold and, combined with non-compressibility, loss of normal wall layering, and surrounding hyperechoic mesenteric fat, confirms acute transmural inflammation. This measurement directly reflects the degree of wall thickening and oedema associated with the inflammatory process. The Radiology in Practice pathway and standard ultrasound protocols use this 6 mm threshold as the primary morphological criterion for diagnosis.
Why each distractor is wrong
Outer-to-outer diameter <6 mm excludes appendicitis and indicates normal appendix: While a diameter <6 mm makes acute appendicitis less likely, it does not absolutely exclude it. Some cases of early or mild appendicitis may present with borderline diameters; the diagnosis is multimodal (clinical, laboratory, and imaging). Additionally, this option reverses the diagnostic logic—the question tests what ≥6 mm means, not what <6 mm means.
Outer-to-outer diameter ≥9 mm is pathognomonic for perforated appendicitis with abscess formation: A diameter of 9 mm indicates acute appendicitis but is not pathognomonic for perforation. Uncomplicated acute appendicitis can present with diameters in this range. The clinical anchor case explicitly states "acute uncomplicated appendicitis" with 9 mm diameter and no free fluid or abscess. Perforation is suggested by additional features (free fluid, abscess, pneumoperitoneum), not diameter alone.
Outer-to-outer diameter measurement is unreliable in females due to hormonal variation and should not be used for diagnosis: This is factually incorrect. Ultrasound diameter measurement is reliable and reproducible regardless of sex. Hormonal variation does not affect appendiceal wall thickness or diameter in a way that invalidates the 6 mm criterion. Gender is not a contraindication to using this measurement.
High-YieldNEET PG
Appendiceal outer-to-outer diameter ≥6 mm on graded-compression ultrasound is the primary diagnostic threshold for acute appendicitis; >6 mm indicates transmural inflammation and oedema.
Radiology in Practice — Right Lower Quadrant Pain Imaging Pathway
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