Radial Neuropathy: This condition, often referred to as "Saturday night palsy" when caused by compression, can lead to acute weakness in the hand and wrist. It affects the radial nerve, which controls the muscles responsible for wrist and finger extension. The lack of trauma and negative X-ray results do not rule out neuropathy, as it can be caused by compression or other non-traumatic means.

Carpal Tunnel Syndrome: Although more commonly associated with median nerve compression, leading to symptoms in the thumb, index, and middle fingers, it can cause significant hand weakness. The presentation might be atypical, especially if the compression affects the nerve at a different level.
Ulnar Neuropathy: This condition affects the ulnar nerve, leading to weakness and numbness, primarily in the little and ring fingers. However, in severe cases, it can contribute to overall hand weakness.
Brachial Plexitis (Neuralgic Amyotrophy): An inflammatory condition affecting the brachial plexus, which can cause acute, severe pain and subsequent weakness in the arm, including the hand and wrist.

Stroke or Transient Ischemic Attack (TIA): Although less likely given the localized nature of the symptoms, a stroke or TIA affecting the motor areas of the brain corresponding to the left hand and wrist could present with acute weakness. This is a medical emergency.
Spinal Cord Injury or Compression: Acute spinal cord compression or injury at the cervical level could lead to weakness in the hand and wrist. Given the potential for serious and irreversible damage, this diagnosis must be considered urgently.
Guillain-Barré Syndrome: An autoimmune disorder that can cause rapid-onset muscle weakness, often starting in the legs but potentially affecting the arms. It's a rare condition but critical to diagnose early due to its potential for respiratory failure.

Plexus Tumors or Cysts: Rarely, tumors or cysts affecting the brachial plexus can cause localized weakness.
Inflammatory Myopathies: Conditions like polymyositis or dermatomyositis, which are inflammatory diseases of the muscles, can cause progressive weakness but are less likely to present acutely in a localized manner.
Vasculitis Affecting Nerves: Certain vasculitic conditions can cause nerve infarction, leading to acute weakness, but these are rare and often associated with systemic symptoms.