A 13-year-old girl has a pigmented mass in the thoracic region at the 10th rib. What is the most likely diagnosis?

In this scenario, the presence of a pigmented mass in a 13-year-old girl at the 10th rib raises suspicion for an accessory nipple, commonly referred to as a supernumerary nipple. Accessory nipples can present as pigmented lesions located along the "milk line," which extends from the axilla through the breast tissue down to the groin area. Since the thoracic region is involved and the patient is of an age where the development of such anatomical variations can be noted, an accessory nipple fits the characteristics described.

Benign pigmented nevi, while common, would typically present differently and are less likely to show up as a prominent mass at the rib level. Melanoma, a more aggressive skin cancer, would be less likely in a young patient presenting with a mass that appears stable. Seborrheic keratosis often occurs in older adults and presents as a wart-like growth, making it an unlikely diagnosis for a pigmented mass in a preadolescent.

Considering the demographic and location, the presentation of a pigmented mass along the ribcage, particularly in a female during early puberty, strongly suggests an accessory nipple rather than the other options.