A patient presents with the skin lesions on the hand as shown in the image. Which of the following is the most likely diagnosis?

Explanation

## Image Findings * Multiple erythematous, edematous papules and plaques are visible on the dorsal aspect of the hand and fingers. * Many lesions exhibit a characteristic **targetoid** or **iris** morphology, featuring concentric rings of color change (e.g., a central dusky or purpuric area, surrounded by a pale edematous ring, and an outermost erythematous halo). * Some lesions appear to be coalescing, forming larger, irregular plaques. ## Diagnosis **Key Point:** The presence of characteristic **targetoid lesions** is pathognomonic for **Erythema Multiforme (EM)**. Erythema multiforme is an acute, self-limited, and sometimes recurrent inflammatory skin condition characterized by its distinctive target lesions. These lesions typically have three concentric zones: a central dusky or purpuric area (which can sometimes be vesicular or bullous), a surrounding pale edematous ring, and an outermost erythematous halo. The lesions are often symmetrically distributed, commonly affecting the extremities (especially palms and soles, and extensor surfaces), and less frequently the trunk. EM is most commonly triggered by infections, particularly **Herpes Simplex Virus (HSV)**, or certain drugs. ## Differential Diagnosis | Feature | Erythema Multiforme (EM) | Urticaria | Psoriasis | Contact Dermatitis | | :------------------ | :------------------------------------------------------ | :------------------------------------------------------- | :-------------------------------------------------------- | :----------------------------------------------------- | | **Lesion Morphology** | Target lesions (3 zones: dusky center, pale ring, red halo) | Wheals (transient, migratory, edematous, erythematous plaques) | Well-demarcated erythematous plaques with silvery scales | Erythema, papules, vesicles, bullae, often linear/geometric | | **Distribution** | Acral (hands, feet, extensor surfaces), symmetric | Widespread, can be anywhere, migratory | Extensor surfaces (elbows, knees), scalp, sacrum | Area of contact with allergen/irritant | | **Duration** | Days to weeks, fixed lesions | < 24 hours per lesion, migratory | Chronic, persistent plaques | Varies, resolves after allergen removal | | **Pathogenesis** | Hypersensitivity reaction (HSV, drugs) | Mast cell degranulation (IgE-mediated or non-IgE) | T-cell mediated autoimmune | Type IV hypersensitivity (allergic) or irritant reaction | ## Clinical Relevance **Clinical Pearl:** While the image depicts a milder form of EM, severe forms like **Erythema Multiforme Major** can involve mucosal surfaces (oral, ocular, genital) and may require systemic treatment. It is crucial to differentiate EM from more severe mucocutaneous reactions such as Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN), which involve significant epidermal detachment and extensive mucosal involvement. ## High-Yield for NEET PG **High-Yield:** The most common trigger for Erythema Multiforme is **Herpes Simplex Virus (HSV) infection**. **Key Point:** Target lesions are the hallmark of Erythema Multiforme. A "typical" target lesion has three concentric zones, while "atypical" target lesions may have only two zones or be raised and edematous without a clear central blister. ## Common Traps **Warning:** Do not confuse the classic targetoid lesions of Erythema Multiforme with less distinct "targetoid" appearances sometimes seen in urticaria pigmentosa (mastocytosis) or certain insect bite reactions. The presence of the classic three-zone morphology is key for diagnosing EM. Also, remember that EM typically has minimal or no epidermal detachment, which helps differentiate it from SJS/TEN. ## Reference [cite:Robbins Basic Pathology Ch 4, Harrison's Principles of Internal Medicine Ch 52]