A 7-year-old boy presents with a 10-day history of palpable purpura on his lower limbs and buttocks following an upper respiratory tract infection. On examination, the purpura is non-blanching and raised. The structure marked **C** in the diagram shows the characteristic predilection site for the rash in this condition. Which of the following best explains why the purpura in Henoch-Schönlein purpura predominantly affects the **extensor surfaces** of the lower extremities and buttocks rather than the trunk and upper limbs?

Explanation

## Why "Dependent areas with increased hydrostatic pressure and gravity-dependent IgA immune complex deposition in small vessels" is right The hallmark distribution of HSP purpura on dependent areas (lower limbs, buttocks) and extensor surfaces reflects the pathophysiology of small-vessel leukocytoclastic vasculitis with IgA immune complex deposition. Gravity and hydrostatic pressure in dependent areas favor accumulation and precipitation of circulating IgA-containing immune complexes in the microvasculature of these regions. The extensor surfaces of the lower extremities and buttocks are both dependent areas and sites of increased mechanical stress, making them preferential sites for immune complex lodgment and subsequent vasculitis. This explains why the trunk and upper limbs are typically spared — they are non-dependent areas with lower hydrostatic pressure. The Nelson 21e and Harrison 21e Ch 363 emphasize this characteristic symmetric distribution on dependent areas as a cardinal feature of HSP diagnosis. ## Why each distractor is wrong - **Higher concentration of IgA-secreting plasma cells in the dermis of extensor surfaces**: While IgA deposition is the pathologic hallmark of HSP, the distribution is not due to regional differences in IgA-secreting plasma cell concentration. The IgA deposition is systemic (found in kidneys, GI tract, joints) but manifests clinically in dependent areas due to hemodynamic factors, not local immune cell populations. - **Preferential IgA deposition in areas of increased mechanical trauma and pressure from clothing**: Although mechanical trauma can exacerbate vasculitis lesions (Koebner phenomenon), the primary distribution of HSP purpura is determined by gravity and hydrostatic pressure in dependent areas, not by clothing pressure or mechanical trauma. This would not explain the consistent sparing of the trunk. - **Increased expression of ICAM-1 and selectin molecules on endothelial cells of lower extremity vessels**: While adhesion molecules are involved in leukocyte recruitment in vasculitis, there is no evidence that ICAM-1 or selectin expression is preferentially upregulated in lower extremity vessels. The distribution pattern is hemodynamic rather than based on differential endothelial adhesion molecule expression. **High-Yield:** HSP purpura distribution = dependent areas + extensor surfaces (gravity-dependent IgA-IC deposition); normal platelets + palpable purpura = vasculitis, not thrombocytopenia. [cite: Nelson 21e; Harrison 21e Ch 363]