Functional neurological disorder (FND): Given the negative imaging, CSF, and serologic work-up, and the subacute onset of paraparesis with numbness, FND is a plausible diagnosis. It is a condition where neurological symptoms cannot be explained by any neurological disease or other medical condition.

Multiple sclerosis (MS): Although the initial work-up is negative, MS can sometimes present with normal initial imaging and CSF analysis, only to show abnormalities on follow-up studies. The subacute onset of neurological symptoms like paraparesis and numbness fits within the spectrum of MS presentations.
Inflammatory or autoimmune myelitis: Conditions like neuromyelitis optica spectrum disorder (NMOSD) or myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) might not always have positive findings on initial evaluations but can cause subacute paraparesis and sensory symptoms.
Spinal cord infarction: Although less common, spinal cord infarction can present subacutely and might not always show clear abnormalities on initial imaging, especially if the infarct is small or in a location that is difficult to visualize.

Spinal dural arteriovenous fistula (SDAVF): This condition can cause progressive myelopathy, including paraparesis and numbness, due to venous congestion in the spinal cord. It is crucial to diagnose SDAVF as it is treatable and can lead to significant disability if missed.
Intramedullary spinal cord tumor: Although imaging is reported as negative, the possibility of a small or iso-intense tumor on MRI cannot be entirely ruled out without a high-quality, contrast-enhanced MRI of the spine. Tumors can cause subacute neurological deterioration.
Vitamin B12 deficiency: A deficiency in vitamin B12 can lead to subacute combined degeneration of the spinal cord, presenting with paraparesis, numbness, and other neurological symptoms. This condition is treatable with supplementation.

Adrenoleukodystrophy: A rare genetic disorder that affects the brain and spinal cord, primarily in males. It can present with progressive myelopathy among other symptoms.
Tropical spastic paraparesis (TSP): Caused by the human T-lymphotropic virus 1 (HTLV-1), TSP is a rare condition that can lead to progressive spastic paraparesis and is more common in certain geographic regions.
Subacute combined degeneration due to copper deficiency: Similar to vitamin B12 deficiency, copper deficiency can lead to myelopathy, although this is exceedingly rare.