Differential Diagnosis for the Abdominal Lesion

The patient presents with a hard "ball" under the skin on her abdomen, which she believes may have originated from an ingrown hair. The lesion has been present for about a year with no visible pus or hair and is not particularly bothersome. Given her history of a similar lesion in the same area that scarred, we can consider the following differential diagnoses:

• Single Most Likely Diagnosis

  • Epidermoid cyst: This is a common, benign cyst that can form when skin cells, rather than shedding, move deeper into the skin and multiply, sometimes as a result of an ingrown hair. The description of a hard "ball" under the skin with no visible pus or hair, and the history of a similar lesion that became a scar, supports this diagnosis.

• Other Likely Diagnoses

  • Sebaceous cyst: Similar to epidermoid cysts but filled with a cheesy, semi-solid material from the sebaceous glands. The absence of visible pus or hair does not rule out a sebaceous cyst, as these can be deep and not visibly connected to the skin surface.
  • Lipoma: A benign tumor composed of fat tissue. It can feel like a hard "ball" under the skin and is generally not bothersome, fitting the patient's description.
  • Dermatofibroma: A small, hard bump that typically develops following a minor injury, most often on the legs but can appear anywhere on the body. It could be considered given the description of a hard "ball," though the history of possibly starting as an ingrown hair is less typical.

• Do Not Miss Diagnoses

  • Squamous cell carcinoma or basal cell carcinoma: Although less likely given the benign appearance and long duration without change, skin cancers can sometimes present as firm nodules under the skin. It's crucial to not miss these diagnoses due to their potential for malignancy and the importance of early treatment.
  • Metastatic disease: Rarely, a metastasis from another cancer can present as a firm nodule under the skin. This would be highly unusual without a known history of cancer but should be considered to avoid missing a potentially life-threatening condition.

• Rare Diagnoses

  • Pilomatrixoma: A rare, benign tumor of the skin that arises from the hair follicle matrix. It can present as a firm, sometimes painful nodule under the skin.
  • Steatocystoma: A rare skin condition characterized by multiple, benign cysts filled with sebum. While less likely given the solitary nature of the lesion, it could be considered in the differential diagnosis.
  • Neurofibroma: A benign tumor of the nerve tissue that can present as a firm nodule under the skin. It's part of neurofibromatosis type 1 (NF1) but can also occur sporadically. The patient's description does not strongly suggest this, but it remains a rare possibility.