Neurofibromatosis type 1 (NF1): Although the presence of a single neurofibroma is not diagnostic on its own, combined with a large brown birthmark (possibly a café-au-lait macule), it raises suspicion for NF1. NF1 is characterized by multiple café-au-lait macules, neurofibromas, freckling in the axillary or inguinal regions, and other features. The diagnosis typically requires two or more of the National Institutes of Health (NIH) criteria, but the presence of these two findings, especially in the context of a large brown birthmark, makes NF1 a consideration.

Isolated Neurofibroma: A single neurofibroma without other stigmata of NF1 could simply be an isolated finding. Many people develop solitary neurofibromas without having NF1.
Café-au-Lait Macule Syndrome: This condition presents with multiple café-au-lait macules without the other features of NF1. It's a benign condition but can be confused with NF1.
Segmental Neurofibromatosis: This is a rare variant of NF1 that affects only one area of the body. If the neurofibroma and birthmark are localized to one segment, this could be a consideration.

Malignant Peripheral Nerve Sheath Tumor (MPNST): Although rare, MPNSTs can arise from neurofibromas in NF1 patients. Early detection is crucial as these tumors are aggressive and have a poor prognosis.
Other Neurocutaneous Syndromes: Conditions like tuberous sclerosis complex or Sturge-Weber syndrome, while distinct, can sometimes present with skin findings that might be confused with those of NF1.

Legius Syndrome: A rare condition characterized by multiple café-au-lait macules, similar to NF1, but without neurofibromas. It's caused by mutations in the SPRED1 gene.
Neurofibromatosis type 2 (NF2): Primarily characterized by bilateral vestibular schwannomas, NF2 can also present with other types of tumors and skin plaques. However, the presence of a single neurofibroma and a large brown birthmark would be unusual for NF2.