The correct answer is:

IgA vasculitis.

Key Point:

The characteristics of a skin lesion (color, location, raised or flat, tender, and size) are important to consider when arriving at a clinical diagnosis.

Explanation:

A systematic evaluation of the characteristics of this patient’s skin lesions leads to the proper diagnosis.

First the reddish nature of the lesions and the fact that they don’t blanch indicates the presence of red blood cells in the superficial layers of the skin. Next, the size and location of the lesions is helpful.

Idiopathic thrombocytopenic purpura (ITP) may be most prominent in the legs but almost never is limited to the lower extremities. Further, the lesions of ITP tend to be uniform, small in size and macular. So, ITP can be ruled out, as can Meningococcemia since the skin lesions of systemic meningococcal infection are similar to those of ITP.

The large size of the lesions is unusual as is the observation that they are papular and somewhat tender. These features suggest an infiltration of cells in the dermal tissues.

The most common cells that infiltrate out of blood, into soft tissue, are white blood cells.

Thrombotic thrombocytopenic purpura (TTP) and IgA vasculitis are the two conditions listed that commonly result in lesions of this size. The lesions of TTP are not often limited to the lower extremities. Further, TTP most often presents with systemic symptoms and signs (i.e. fever, thrombosis, neurologic signs), which are features that are not present in this case.

Thus, we come, by exclusion, to the diagnosis of Immune complex A (Ig A) vasculitis.

This condition, also known as Henoch-Schonlein purpura, is one of a number of small vessel vasculitides which include antineutrophil cytoplasmic antibody (ANCA) disease, microscopic polyangiitis, Wegener’s granulomatosis, cryoglobulinemic vasculitis and immune complex small vessel vasculitis. The immune deposits may be limited to a single organ such as the skin or take the form of a systemic disease with renal and sometimes hepatic involvement.

Confirmation of the diagnosis requires a skin biopsy showing the deposition of IgA.

IgA vasculitis most often follows an infection or the use of a new antibiotic for a recent infection. A reasonable explanation for the vasculitis in this case is the patient’s history of a lower urinary tract infection treated with fosfomycin. If so, it will subside spontaneously and not recur. In cases where lesions persist or new lesions regular appear, large doses of corticosteroids and/or modern immune-suppressive drugs may help.

This question shows how a systematic evaluation of the characteristics of a lesion or lesions -- location, size, type of lesion (macular, papular, pustular), color, and presence or absence of tenderness -- can lead to a logical diagnosis, either directly or by elimination of other diagnostic considerations.

References:

Mcinnes I, O’Dell JR. The Systemic Vasculitides. Goldman-Cecil Medicine. 2020; 254: 1750-1758.e2

Clinical Overview - Henoch-Schönlein purpura. Elsevier Point of Care. Updated November 29, 2018