Lumbar Radiculopathy: This condition, often due to a herniated disc, can cause pain that radiates from the hip down to the ankle. The pain typically worsens with positions that increase the lumbar lordosis, such as sitting, and improves with standing or walking, which reduces the pressure on the affected nerve root.

Sciatica: Similar to lumbar radiculopathy, sciatica involves pain along the sciatic nerve, which runs from the lower back down to the legs. It can be exacerbated by sitting or lying down and may improve with standing or movement.
Piriformis Syndrome: This condition occurs when the piriformis muscle, located in the buttock region, compresses or irritates the sciatic nerve. It can cause pain that radiates down the leg and may worsen with sitting or activities that involve pivoting on the leg.
Peripheral Artery Disease (PAD): While more commonly associated with claudication (pain in the legs or arms during exercise), PAD can cause pain at rest, especially in severe cases. However, the pain typically worsens with walking and improves with rest, which is somewhat opposite to the described symptoms.

Cauda Equina Syndrome: A serious condition that occurs when the nerves in the spinal canal are compressed, leading to pain, numbness, and potentially permanent nerve damage if not promptly treated. Symptoms can include severe low back pain, sciatica, and changes in bladder or bowel function.
Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the legs can cause pain, swelling, and warmth in the affected leg. While DVT typically does not improve with standing, it is a critical condition to rule out due to the risk of pulmonary embolism.
Osteonecrosis (Avascular Necrosis): This condition involves the death of bone tissue due to a lack of blood supply. It can cause pain in the hip, knee, or ankle, depending on the affected bone, and may worsen with weight-bearing activities.

Chronic Regional Pain Syndrome (CRPS): A condition characterized by intense burning or aching pain in the hands or feet after an injury or trauma. It can also affect the legs and may worsen with certain positions or activities.
Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Some types can cause joint hypermobility and pain, which might fit the described symptoms in rare cases.
Neuropathic Pain Conditions: Such as complex regional pain syndrome or certain types of neuropathy, which can cause chronic pain in the legs and may have variable responses to position changes.