Conversion Disorder: This condition, also known as functional neurological symptom disorder, presents with neurological symptoms that cannot be explained by medical conditions. Given the normal imaging and studies, and the presence of persistent weakness, it's a plausible explanation. The symptoms are real to the patient but are not consistent with any identifiable neurological disease.

Multiple Sclerosis (MS) with a Normal MRI: Although MS typically shows lesions on MRI, there are cases where the MRI can appear normal, especially in the early stages of the disease. The clinical presentation of weakness, particularly if it's unilateral and involves both upper and lower limbs, could suggest MS.
Peripheral Neuropathy: Certain types of peripheral neuropathy can cause weakness. While some forms might show abnormalities on electromyography (EMG) or nerve conduction studies (NCS), others might not, especially in early stages or if the neuropathy is patchy.
Myasthenia Gravis: An autoimmune disease that affects the neuromuscular junction, leading to fluctuating muscle weakness. It can sometimes present with limb weakness, and while there are diagnostic tests like acetylcholine receptor antibody tests and single-fiber EMG, these might not always be positive, especially early in the disease.

Spinal Cord Infarction or Small Vessel Disease: These conditions might not always show up on standard MRI scans, especially if the infarct is small or in a location that's hard to visualize. They are critical to diagnose because they can lead to significant and permanent neurological deficits if not promptly treated.
Lyme Disease: Neurological manifestations of Lyme disease can include weakness, among other symptoms. It's a diagnosis that should be considered, especially in endemic areas, because it's treatable with antibiotics.
Vitamin B12 Deficiency: Can cause subacute combined degeneration of the spinal cord, leading to weakness, among other symptoms. It's easily treatable with supplementation but can lead to irreversible damage if not addressed.

Mitochondrial Myopathies: A group of disorders that affect the mitochondria, leading to muscle weakness among other symptoms. They can be difficult to diagnose and might not show up on routine tests.
Inflammatory Myopathies (e.g., Polymyositis, Dermatomyositis): These are autoimmune conditions that cause muscle inflammation and weakness. While they typically have elevated muscle enzymes and specific findings on muscle biopsy, there can be cases where these tests are normal or not significantly abnormal.
Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): An autoimmune disorder that can cause progressive weakness and impaired sensory function. It might not always show the typical demyelination pattern on NCS and can be a diagnosis of exclusion.