S1 Radiculopathy: This condition is the most likely cause due to the symptoms of pain in the right lateral leg and posterior calf, dysesthesia, and a reduced right ankle reflex. The S1 nerve root is responsible for sensation in the posterior calf and lateral aspect of the foot, and its impairment can lead to these specific symptoms.

Peroneal Nerve Compression: Compression of the peroneal nerve can cause pain and dysesthesia in the lateral leg. However, it typically spares the posterior calf and might not directly affect the ankle reflex.
Tibial Nerve Compression: Compression of the tibial nerve can cause symptoms in the posterior calf but might not fully explain the lateral leg involvement unless the compression occurs at a level that affects both peroneal and tibial nerve distributions.
Lumbar Disc Herniation: A herniated disc, particularly at L5-S1, can compress the S1 nerve root, leading to similar symptoms. However, the presentation might be more complex, involving other nerve roots or requiring additional symptoms for diagnosis.

Cauda Equina Syndrome: Although less likely, this is a medical emergency that requires immediate attention. It involves compression of the cauda equina (nerve roots in the lumbar spine) and can present with bilateral leg pain, sensory disturbances, and significant impairment of bladder and bowel function. Missing this diagnosis could lead to permanent neurological damage.
Deep Vein Thrombosis (DVT): DVT can cause leg pain and swelling but typically does not cause dysesthesia or affect reflexes directly. However, it's a critical condition due to the risk of pulmonary embolism.
Peripheral Artery Disease: While more commonly associated with claudication (pain on walking that improves with rest), severe cases could potentially cause pain at rest and sensory changes, especially if there's significant ischemia.

Neuropathy due to Vitamin Deficiency: Deficiencies in vitamins like B12 can cause neuropathy, which might present with dysesthesia and reflex changes. However, this would typically be a more generalized condition rather than localized to one leg.
Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): An autoimmune disorder that can cause nerve root symptoms, including pain, weakness, and sensory changes. It's less common and would likely have a more widespread presentation.
Tumors (e.g., Schwannoma, Neurofibroma): Rarely, tumors along the nerve roots or peripheral nerves can cause localized pain, dysesthesia, and reflex changes. These would be unusual causes of the described symptoms.