Differential Diagnosis for Large Brown Horny Skin Lesion on Chest of Elderly Male

• Single most likely diagnosis
  • Seborrheic keratosis: This is the most common diagnosis due to its high prevalence in elderly individuals, and the description of a large brown horny skin lesion is consistent with the typical presentation of seborrheic keratosis.
• Other Likely diagnoses
  • Solar lentigo: Given the location on the chest, which is an area exposed to the sun, and the age of the patient, solar lentigo is a plausible diagnosis. However, it tends to be flatter and less horny than seborrheic keratosis.
  • Dermatofibroma: Although less common than seborrheic keratosis, dermatofibromas can present as firm, brown lesions and should be considered, especially if the lesion is more nodular.
  • Keratoacanthoma: This is a low-grade malignancy that can resemble a large, horny lesion. It's less common but should be considered due to its potential for local destruction if left untreated.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Malignant melanoma: Although less likely than benign lesions, melanoma can present in various ways, including as a large, pigmented lesion. Given the potential lethality of melanoma, it must always be considered in the differential diagnosis of any new or changing skin lesion.
  • Squamous cell carcinoma: This type of skin cancer can present as a firm, horny lesion, especially in sun-exposed areas. It's crucial to not miss this diagnosis due to its potential for metastasis.
• Rare diagnoses
  • Epidermal nevus: A congenital or acquired lesion that can appear as a large, brown, verrucous plaque. It's less common and might not be the first consideration but should be kept in mind for lesions with a distinctive appearance or distribution.
  • Porokeratosis: A rare disorder of keratinization that can present with atypical, large, brown lesions. It's an important consideration for lesions that do not fit the typical presentation of more common diagnoses.