What glomerular abnormality is likely present in a 17-year-old boy with non-blanching rash, swollen knee, and visible haematuria?

In the context of a 17-year-old boy presenting with a non-blanching rash, swollen knee, and visible hematuria, the most consistent glomerular abnormality would be the mesangial deposition of IgA, which is commonly associated with IgA nephropathy (Berger's disease). This condition is characterized by the deposition of IgA in the mesangial areas of the glomeruli, which can lead to glomerular inflammation and is often associated with episodes of visible hematuria, especially following infections or physical stress.

The presence of a non-blanching rash in this scenario raises the suspicion of a systemic condition, possibly vasculitis or another immune-mediated process, which frequently correlates with IgA nephropathy. The knee swelling suggests potential joint involvement, common in conditions that affect the systemic vascular system. Thus, the combination of these symptoms strongly points towards IgA nephropathy due to its classic presentation.

This choice aligns well with the clinical picture, making it the most appropriate answer in this case.