Subungual melanoma: Although rare, given the patient's history of a facial Spitz nevus, which is a type of melanocytic nevus, there might be an increased predisposition to melanocytic lesions, including melanoma. The presence of a subungual dark lesion, especially if it is new, changing, or exhibits characteristics such as the ABCDE rule (Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolving), warrants consideration of melanoma.

Subungual hematoma: This is a common condition that can cause a dark appearance under the nail due to blood accumulation. It often results from trauma but can also occur spontaneously.
Subungual nevus: Benign melanocytic nevi can occur under the nail and present as dark lesions. Given the patient's history of a Spitz nevus, this could be a consideration.
Melanonychia: This refers to a brown or black pigmentation of the nail plate due to an increase in melanin production. It can be caused by a variety of factors, including trauma, infection, or systemic conditions.

Subungual squamous cell carcinoma: Although less common than melanoma, this type of skin cancer can present under the nail and is important not to miss due to its potential for aggressive behavior.
Kaposi's sarcoma: In immunocompromised patients, Kaposi's sarcoma can present as subungual lesions. Given the patient's age and no mentioned immunocompromised state, this is less likely but should be considered in the appropriate clinical context.
Glomus tumor: A rare, benign tumor of the glomus body that can occur under the nail, presenting as a painful, blue-purple lesion.

Subungual keratoacanthoma: A rare, low-grade malignancy that can mimic squamous cell carcinoma but often has a more benign course.
Subungual metastasis: Metastatic disease to the subungual region from other primary cancers is extremely rare but should be considered in patients with a known history of cancer.
Subungual blue nevus: A rare type of melanocytic nevus that can present as a blue-gray lesion under the nail.