L2-L4 Radiculopathy: This is the most likely diagnosis because the patellar reflex is primarily mediated by the L2-L4 nerve roots. A lesion or compression affecting these roots could lead to a weakened patellar reflex on the affected side, while sparing the Achilles reflex, which is mediated by the S1 nerve root.

Femoral Neuropathy: Damage to the femoral nerve, which innervates the quadriceps muscle responsible for the patellar reflex, could result in a weakened reflex. This would not affect the Achilles reflex.
Lumbar Plexopathy: A condition affecting the lumbar plexus, from which the femoral nerve originates, could also lead to a weak patellar reflex if the nerves supplying the quadriceps are involved.

Spinal Cord Compression or Lesion: Although less common, a spinal cord lesion (e.g., tumor, abscess, or hematoma) at the level of the lumbar spine could cause a unilateral weak patellar reflex. Missing this diagnosis could have significant consequences, including paralysis and loss of bladder and bowel function.
Multiple Sclerosis: An early or mild presentation of multiple sclerosis could involve the spinal cord and lead to asymmetric reflex changes, including a unilaterally weakened patellar reflex.

Neuromuscular Junction Disorders (e.g., Myasthenia Gravis): While these conditions typically present with more widespread and fluctuating weakness, a rare, localized form could potentially affect the quadriceps muscle, leading to a weak patellar reflex.
Polymyositis or Dermatomyositis: Inflammatory myopathies could cause muscle weakness, potentially affecting the quadriceps and resulting in a weakened patellar reflex, although this would typically be part of a more generalized condition.