Differential Diagnosis for Adult Male with Papules on Thumb

Single Most Likely Diagnosis

• Hyperkeratosis: A common condition characterized by thickening of the skin, often caused by repeated friction or pressure. The location on the thumb and the fact that it's more noticeable in hot months (possibly due to increased sweating or friction) support this diagnosis. The minimal pruritus and lack of inflammation, redness, vesicles, or drainage also align with hyperkeratosis.

Other Likely Diagnoses

• Eczema (Dyshidrotic): Although the patient mentions minimal pruritus and no vesicles, a mild form of dyshidrotic eczema could still be considered, especially given the seasonal variation and the fact that symptoms can vary widely among individuals.
• Wart (Verruca): Warts can present as papules and are common on the hands. They can be more noticeable or spread in certain conditions, such as warm weather, due to increased moisture which might facilitate viral transmission.
• Keratoacanthoma: A low-grade, or sometimes considered benign, tumor of the skin that can resemble a papule. It often grows rapidly but can stabilize and even regress. The lack of inflammation or other symptoms doesn't rule out this possibility entirely.

Do Not Miss Diagnoses

• Squamous Cell Carcinoma (SCC): Although less common than other skin cancers, SCC can present as a firm, rough-surfaced papule and is crucial to diagnose early due to its potential for metastasis. The fact that it's been present for a year without significant change somewhat lowers the likelihood, but it cannot be overlooked.
• Mycobacterial Infection: Certain mycobacterial infections, like those caused by Mycobacterium marinum, can cause skin lesions (often after exposure to contaminated water) that might resemble papules. These infections are important to identify due to their need for specific antibiotic treatments.

Rare Diagnoses

• Keratosis Pilaris: While more common on the arms and legs, keratosis pilaris could theoretically occur on the hands, presenting as small, rough papules. However, its occurrence on the thumb, especially with a seasonal pattern, would be unusual.
• Darier's Disease: A rare genetic disorder that affects the skin, presenting with greasy, crusted papules. The description provided doesn't closely match Darier's disease, but it's an example of a rare condition that could, in some forms, present with papular lesions.