Meralgia Paresthetica: This condition is characterized by numbness, tingling, and burning pain in the outer part of the thigh. However, given the patient's symptoms of severe muscle spasms in the inner thigh triggered by standing after lying in a prone position, a variant or related condition affecting the inner thigh could be considered. The prone position may cause compression or irritation of the nerves supplying the inner thigh muscles, leading to spasms upon standing.

Lumbar Radiculopathy: Compression or irritation of the lumbar nerve roots (especially L2-L4) can cause pain, numbness, and muscle spasms in the thigh. The symptoms could be exacerbated by positions that increase lumbar lordosis or nerve root compression, such as standing after lying prone.
Peripheral Artery Disease (PAD): Although PAD typically presents with claudication (pain in the legs or arms during exercise), severe cases could potentially cause muscle spasms due to ischemia, especially if the patient has significant atherosclerotic disease.
Diabetic Amyotrophy: This condition, also known as diabetic lumbosacral radiculoplexus neuropathy, can cause severe pain, muscle weakness, and atrophy, typically in the proximal muscles of the lower limbs. Muscle spasms could be a part of the clinical presentation.

Abdominal Aortic Aneurysm (AAA): Although AAA typically presents with abdominal pain, a large aneurysm could compress or embolize to the iliofemoral arteries, causing severe leg symptoms, including muscle spasms. This is a medical emergency.
Spinal Stenosis: This condition can cause neurogenic claudication, which might include muscle spasms in the thighs upon standing or walking. It's crucial to consider spinal stenosis due to its potential for significant disability and the availability of surgical interventions.

Neuromyotonia: A rare condition characterized by muscle stiffness, cramps, and delayed relaxation of the muscles after voluntary contraction. It could potentially cause severe muscle spasms, although it's less likely given the specific trigger of standing after a prone position.
Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): An autoimmune disorder that can cause a wide range of neurological symptoms, including muscle weakness and potentially spasms. However, the specific pattern described would be unusual for CIDP.