Stroke (specifically, a lacunar stroke or a small vessel disease affecting the motor pathways): This is the most likely diagnosis due to the acute onset and asymmetrical nature of the paraparesis. The lack of sensory, bowel, or bladder involvement suggests a lesion that is primarily affecting the motor pathways, which is consistent with certain types of stroke.

Spinal Cord Infarction: Although less common than stroke, spinal cord infarction can present with acute onset of asymmetrical paraparesis. The absence of sensory, bowel, or bladder symptoms might suggest a more localized lesion affecting primarily the anterior spinal artery territory, which supplies the motor tracts.
Multiple Sclerosis (MS) or Acute Disseminated Encephalomyelitis (ADEM): These demyelinating diseases can present with a wide range of neurological symptoms, including acute onset of asymmetrical weakness. However, they typically involve sensory symptoms as well, but in some cases, the presentation might be predominantly motor.
Infectious or Inflammatory Myelitis: Conditions like transverse myelitis can cause acute weakness, but they usually involve sensory symptoms and sometimes bowel and bladder dysfunction. However, early in the course or in milder cases, the presentation might be more limited.

Spinal Epidural Abscess or Hematoma: These conditions are medical emergencies that can present with acute onset of weakness, including asymmetrical paraparesis. Even though sensory, bowel, or bladder involvement is often present, it's crucial to consider these diagnoses due to their potential for serious complications and the need for urgent intervention.
Cauda Equina Syndrome: Although typically associated with bowel, bladder, and sexual dysfunction, early or mild cases might present with less typical symptoms, including asymmetrical paraparesis without clear sensory involvement.

Neuromyelitis Optica Spectrum Disorder (NMOSD): This is an autoimmune inflammatory condition that predominantly affects the optic nerve and spinal cord. While it more commonly presents with optic neuritis and transverse myelitis, rare cases could have a more limited presentation.
Vitamin B12 Deficiency: A deficiency in vitamin B12 can lead to subacute combined degeneration of the spinal cord, which might present with weakness, but this is typically a more gradual process and often involves sensory symptoms.
Inherited Ataxias or Spastic Paraplegias: These are a group of rare genetic disorders that can cause progressive weakness and spasticity of the legs. While they are not typically characterized by acute onset, some forms could present in adulthood with a more rapid progression that might mimic an acute onset.