A 53-year-old woman presents with a small, non-tender, smooth lump in her neck. What is the most likely diagnosis?

In evaluating the presentation of a small, non-tender, smooth lump in the neck of a 53-year-old woman, the most likely diagnosis is a sebaceous cyst. Sebaceous cysts, also known as epidermoid cysts, are common, benign growths that can appear as smooth lumps under the skin. They typically present as non-tender masses and can be located anywhere on the body, including the neck.

The characteristics of benign cysts, including their non-tender nature and smooth surface, align well with the description provided in the scenario. These cysts originate from sebaceous glands and can become prominent if they contain keratin or other debris.

In contrast, while thyroid nodules and goiter might present similarly, they often have different clinical implications and characteristics. Thyroid nodules can vary in consistency, and goiter usually involves enlargement of the entire thyroid gland, not just a discrete lump. Malignant lymphoma typically presents with other signs, such as tenderness, systemic symptoms, or involvement of multiple lymph nodes, which are not indicated in this case.

With these factors in mind, the identified features of the lump strongly suggest that it is a sebaceous cyst, making it the most appropriate diagnosis for this situation.