Lumbar Radiculopathy: This is the most likely diagnosis due to the common occurrence of lower back problems that can radiate pain and weakness to the lower limbs. The proximity of the lumbar spine to the lower limbs and the frequency of conditions such as herniated discs, spinal stenosis, and spondylolisthesis make this a probable cause.

Peripheral Neuropathy: Conditions like diabetes, alcohol abuse, or vitamin deficiencies can lead to peripheral neuropathy, causing pain, numbness, and weakness in the lower limbs.
Musculoskeletal Injuries: Strains, sprains, or overuse injuries can cause significant pain and weakness, especially if they involve major muscle groups or tendons in the lower limbs.
Osteoarthritis: Degenerative joint disease can lead to pain and weakness, particularly in weight-bearing joints like the hips, knees, and ankles.

Acute Ischemia: Sudden onset of severe pain and weakness could indicate an acute ischemic event, such as an arterial embolism or thrombosis, which requires immediate medical attention.
Spinal Cord Compression: Conditions like spinal tumors, epidural abscess, or hematoma can compress the spinal cord, leading to rapid progression of neurological deficits, including pain and weakness in the lower limbs.
Guillain-Barré Syndrome: An autoimmune disorder that can cause rapid onset of muscle weakness, often starting in the lower limbs and ascending, which can lead to respiratory failure if not promptly treated.

Piriformis Syndrome: Compression of the sciatic nerve by the piriformis muscle can cause pain and weakness in the lower limb, though it is less common than other causes of sciatica.
Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): A rare autoimmune disorder that can cause progressive weakness and loss of sensation in the limbs.
Vasculitis: Inflammation of the blood vessels can lead to ischemia and subsequent pain and weakness in the affected limbs, though this is a less common cause of lower limb symptoms.