Differential Diagnosis for Dark Square Spot on Lower Back
- Single most likely diagnosis
- Melanocytic nevus (mole): A common, benign skin lesion that can appear as a dark spot, often round or oval but can sometimes appear square due to the skin's surface characteristics or the way the lesion grows.
- Other Likely diagnoses
- Seborrheic keratosis: A benign skin growth that can appear as a dark, wart-like spot and can sometimes have a square shape due to its growth pattern.
- Dermatofibroma: A small, round or oval growth that can appear after a minor injury, sometimes presenting as a dark spot, though less commonly square.
- Cafe-au-lait macule: A benign skin patch that can be darker than the surrounding skin, though typically lighter than a melanocytic nevus and can vary in shape.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
- Melanoma: Although less common, a new or changing dark spot, especially one that is asymmetric, has irregular borders, color variation, diameter larger than 6mm, or is evolving, could be a sign of skin cancer.
- Metastatic lesion: In rare cases, a dark spot could be a metastasis from an internal malignancy, which would be critical to identify early.
- Rare diagnoses
- Blue nevus: A type of melanocytic nevus that can appear more blue or black due to the depth of the melanin in the skin, and while typically round, could appear square.
- Mongolian spot: A congenital melanocytic lesion that appears as a blue or blue-gray flat spot, most common in infants, which can sometimes appear on the lower back and may fade with age.
- Tattoo or pigment implantation: Though not a medical condition per se, a dark square spot could be due to intentional or accidental tattooing or pigment implantation in the skin.