IgA Vasculitis Presenting As Isolated Monoarthritis Mimicking Transient Synovitis: A Diagnostic Challenge With Delayed Purpura.

IgA vasculitis (IgAV), previously known as Henoch-Schönlein purpura, is a small-vessel vasculitis that may involve the skin, joints, gastrointestinal tract, and kidneys. While purpura is a key clinical feature, presentations without rash at onset can occur and may lead to diagnostic uncertainty. We report the case of a two-year-old boy who presented with a three-day history of acute right knee pain, swelling, and reduced weight-bearing ability following a recent febrile respiratory illness. Ultrasound of the knee demonstrated mild periarticular fluid, and a provisional diagnosis of transient synovitis was made. No rash or systemic features were present at initial presentation. Two days later, the child developed a progressive rash over the lower limbs with erythematous lesions and evolving purpuric features, along with right elbow swelling and intermittent abdominal pain. These findings prompted further evaluation and led to a clinicoradiological diagnosis of IgAV. Abdominal ultrasound excluded intussusception and demonstrated mesenteric lymphadenitis. Laboratory investigations showed mild inflammatory changes and reactive thrombocytosis, while renal function and serial urinalysis remained normal. The patient was managed conservatively with close monitoring and safety-netting, without corticosteroid therapy. Symptoms gradually resolved, and no renal involvement was identified during follow-up. This case highlights an atypical presentation of IgAV in which isolated monoarthritis preceded the development of purpura, emphasizing the importance of diagnostic vigilance and serial clinical reassessment.