**Core Concept**
The question revolves around the differential diagnosis of a skin lesion in a 50-year-old woman. The key concept here is the assessment of a skin lesion's characteristics to determine its underlying cause. In this case, the question requires the evaluation of a skin lesion's appearance, location, and other clinical features to narrow down the differential diagnosis.

**Why the Correct Answer is Right**
The correct answer is Metastasis. The most common skin lesions that are metastatic in origin are from breast, melanoma, and lung cancers. The presence of a new skin lesion in a 50-year-old woman, especially if it is pigmented or ulcerated, raises the suspicion of metastasis. The skin lesions can be the first presentation of an underlying malignancy or can occur as a result of metastasis from a primary tumor. The clinical features of metastatic skin lesions can vary widely, but they often present as solitary or multiple, well-defined, or ill-defined, pigmented or non-pigmented lesions.

**Why Each Wrong Option is Incorrect**
**Option B:** Onychomycosis is a fungal infection of the nail, which is not a skin lesion. It does not match the description of the lesion in question.
**Option C:** Psoriasis is a chronic skin condition characterized by erythematous, scaly plaques. While it can present with skin lesions, it does not typically present as a solitary, pigmented or ulcerated lesion.
**Option D:** Thromboangiitis obliterans is a rare disease characterized by inflammation of blood vessels, leading to skin lesions. However, it typically presents with painful lesions on the extremities, which is not consistent with the description in the question.

**Clinical Pearl / High-Yield Fact**
When evaluating skin lesions, it is essential to consider the patient's age, sex, and medical history. A high index of suspicion for metastasis is necessary in patients with a history of malignancy or in those with new or changing skin lesions.

**✓ Correct Answer: A. Metastasis**