Question: Can you identify this condition?

Gülsüm Gençoğlan; Dilek Toprak

Fibrous lesion with central pit and hyperpigmentation in the lumbosacral area with surrounding hypertrichosis

Magnetic resonance image of the lumbosacral area: A) Posterior elements of the spine are not closed at the S1 level, but are instead fixed to the dermal sinus tract; B) Inside the spinal cord, low-volume hydromyelia can be observed at the L2-L3 level.

An 11-year-old girl presented with an asymptomatic hairy lesion on her lower back, which had been there since birth and gradually enlarged. She did not have a history of enuresis, a defecation problem, or weakness of the lower extremities, and was generally healthy. None of her relatives had a similar lesion.

Dermatologic examination showed a 4-cm × 4-cm fibrous lesion, with a central pit and hyperpigmentation, in the lumbosacral area. It was mildly tender when palpated, with surrounding hypertrichosis. Physical examination, including neurologic assessment, did not reveal any abnormal findings. Routine hematologic and biochemical parameters were normal. Magnetic resonance imaging of the area revealed the following: The filum terminale was thick and short, while the conus medullaris ended at the third and fourth lumbar vertebrae (L3-L4) level. The dermal sinus tract at the first sacral vertebra (S1) level included skin and subcutaneous adipose tissue, but did not extend into the spinal canal. At the L2-L3 level, inside the spinal cord, low-volume hydromyelia was observed. Posterior elements were not closed at S1. This localization was fixed to the dermal sinus.

The most likely diagnosis is