Peripheral neuropathy (e.g., peroneal or femoral neuropathy): This is a common cause of decreased reflexes, especially if the decrease is unilateral. Peripheral neuropathy can result from various factors such as trauma, compression, or systemic diseases like diabetes.

L2-L4 radiculopathy: Nerve root compression or damage at the L2-L4 levels can affect the reflex arc of the patellar reflex, leading to decreased reflexes on the affected side.
Spinal stenosis or lumbar spondylosis: These conditions can cause compression of the spinal cord or nerve roots, potentially affecting reflexes.
Stroke or cerebral vasculature issues: Although less common for isolated decreased patellar reflexes, a stroke affecting the motor pathways can lead to changes in reflexes.

Multiple sclerosis: An autoimmune disease that can affect any part of the central nervous system, including the spinal cord, leading to a wide range of neurological symptoms, including changes in reflexes.
Spinal cord injury or compression: Conditions such as spinal cord tumors, abscesses, or herniated disks can compress the spinal cord, leading to neurological deficits, including decreased reflexes.
Guillain-Barré Syndrome: A rare autoimmune disorder where the body's immune system mistakenly attacks part of its peripheral nervous system, which can lead to muscle weakness and changes in reflexes.

Tay-Sachs disease or other storage diseases: Rare genetic disorders that can lead to neurological deterioration, including changes in reflexes.
Subacute combined degeneration (vitamin B12 deficiency): A condition that affects the spinal cord and can lead to changes in reflexes, among other neurological symptoms.
Inherited neuropathies (e.g., Charcot-Marie-Tooth disease): A group of disorders that affect the peripheral nerves, leading to muscle weakness and changes in reflexes.