This patient has erythroderma, which is a term used to describe erythematous inflammation of at least 80% to 90% of the skin surface and is a dermatologic urgency. Erythroderma is most often due to an uncontrolled existing dermatosis such as cutaneous T-cell lymphoma, graft-versus-host disease, psoriasis, or pityriasis rubra pilaris. Medications are the second most common cause; thus, taking a history of current and recently discontinued medications is also important. Men are affected more often than women, and the average age of onset is 55 years. Although the diagnosis of erythroderma is usually straightforward, in some patients it may be difficult to determine the cause, particularly in patients without a known preexisting condition that may be responsible. In these patients, a combination of clinical evaluation and careful clinicopathologic correlation is needed. Owing to compromise of the skin barrier, affected patients are at risk for dehydration, electrolyte abnormalities, protein loss, heat loss, and infection. Peripheral edema, erosions from severe pruritus, scaling, and lymphadenopathy are common findings.

Angioedema is soft-tissue swelling that may be mast cell or bradykinin mediated. It may occur independently, with urticaria, or may be associated with anaphylaxis. Angioedema tends to affect less dense connective tissue, including the face, lips, throat, and extremities, but is not associated with erythroderma. This patient's presentation is therefore not consistent with angioedema.

Drug-induced subacute cutaneous lupus erythematosus (SCLE) can be triggered by hydrochlorothiazide. Other medications that can cause this eruption are calcium channel blockers, ACE inhibitors, statins, proton-pump inhibitors, and tumor necrosis factor α inhibitors. Although this patient is taking hydrochlorothiazide, the appearance of his rash is not consistent with SCLE. Drug-induced SCLE typically presents as annular pink-red patches with fine scale, predominantly affecting the photodistributed areas of the upper chest, back, and shoulders.

Pityriasis refers to flaking of the skin, and a number of cutaneous conditions incorporate this descriptive term into their name. Pityriasis rosea (PR) is a relatively common inflammatory reaction of the skin and may be caused by a viral infection; however, a specific virus causing this condition has not been identified. It generally occurs in patients younger than 30 years old and does not cause an erythrodermic eruption. Typically, PR begins as a single annular patch or plaque with fine scaling (the so-called herald patch). About 1 week later many smaller pink macules, patches, and papules erupt over the trunk and proximal extremities.