Lumbar Radiculopathy: This condition, often due to a herniated disc, can cause proximal leg weakness by compressing nerve roots that control muscle function in the legs. The proximity of the lumbar spine to the proximal leg muscles and the common occurrence of lumbar disc issues make this a leading consideration.

Peripheral Neuropathy: Conditions like diabetic neuropathy can lead to proximal leg weakness among other symptoms, affecting the nerves that supply the muscles.
Myopathies: Diseases affecting the muscles directly, such as polymyositis or dermatomyositis, can cause proximal muscle weakness, including in the legs.
Guillain-Barré Syndrome: An autoimmune disorder that can lead to rapid-onset muscle weakness, often starting in the proximal muscles of the legs.

Spinal Cord Compression: Conditions like spinal stenosis, tumors, or abscesses can compress the spinal cord, leading to leg weakness. Missing this diagnosis could result in permanent neurological damage.
Cauda Equina Syndrome: A serious condition where the nerves in the spinal canal are compressed, leading to leg weakness, among other symptoms. Prompt recognition and treatment are crucial to prevent long-term damage.
Vasculitis: Inflammation of blood vessels can lead to ischemia of muscles and nerves, causing weakness. Certain types, like giant cell arteritis, can have serious consequences if not treated promptly.

Inclusion Body Myositis: A rare inflammatory muscle disease that can cause progressive proximal leg weakness.
Mitochondrial Myopathies: A group of rare disorders affecting the mitochondria, leading to muscle weakness, including in the proximal legs.
Neurosyphilis: A rare condition, especially in the modern antibiotic era, but can cause a range of neurological symptoms, including leg weakness, if left untreated.