Stroke (specifically, a lacunar stroke or a stroke affecting the internal capsule or corona radiata): This is the most likely diagnosis because the pattern of weakness (left upper limb and right lower limb) suggests a lesion in the brain that affects the motor pathways, which could be due to a vascular event like a stroke. The internal capsule, being a compact structure with a high concentration of motor fibers, is a common site for such lesions.

Multiple Sclerosis (MS): MS is an autoimmune disease that can cause lesions in the central nervous system, leading to a variety of neurological symptoms, including limb weakness. The pattern of left upper limb and right lower limb weakness could be consistent with a relapse in MS, especially if there are other symptoms such as optic neuritis, sensory disturbances, or bladder and bowel dysfunction.
Spinal Cord Lesion (e.g., spinal cord infarction, tumor, or trauma): A lesion in the spinal cord, particularly at the level of the cervical or thoracic spine, could cause weakness in the limbs, depending on the level and extent of the lesion. The pattern of weakness could suggest a lesion affecting the lateral corticospinal tract, which carries motor signals from one side of the brain to the opposite side of the body below the level of the lesion.
Brain Tumor: A tumor in the brain, depending on its location, could cause focal neurological deficits, including limb weakness. Tumors in the motor areas of the brain or in the pathways leading from these areas could result in the described pattern of weakness.

Subarachnoid Hemorrhage: Although less common, a subarachnoid hemorrhage (bleeding into the space surrounding the brain) can present with sudden onset of neurological deficits, including limb weakness. This condition is medical emergency and requires immediate attention.
Spinal Epidural Abscess: An abscess in the spinal epidural space can compress the spinal cord, leading to neurological deficits, including limb weakness. This condition is a medical emergency due to the risk of rapid progression to spinal cord compression and paralysis.
Cervical Spondylotic Myelopathy: This condition, caused by degenerative changes in the cervical spine, can lead to compression of the spinal cord and result in limb weakness, among other symptoms. It's crucial to identify and treat this condition to prevent further neurological deterioration.

Neurosyphilis: In its tertiary stage, syphilis can cause a variety of neurological symptoms, including limb weakness, due to inflammation and damage to the central nervous system.
Vasculitis (e.g., Primary Central Nervous System Vasculitis): Vasculitis affecting the blood vessels of the brain can lead to ischemic or hemorrhagic lesions, resulting in focal neurological deficits like limb weakness.
Mitochondrial Disorders: Certain mitochondrial disorders can affect the nervous system and muscles, leading to weakness among other symptoms. These disorders are genetic and often have a complex presentation.