Motor Neuron Disease: This condition, which includes amyotrophic lateral sclerosis (ALS), is known for affecting both the upper and lower motor neurons. The combination of absent ankle reflexes (indicating lower motor neuron involvement) and an upgoing plantar response (Babinski sign, indicating upper motor neuron involvement) makes motor neuron disease a strong candidate for this presentation.

Vitamin B12 Deficiency: While the question mentions Vitamin B6, Vitamin B12 deficiency is more commonly associated with neurological symptoms, including absent reflexes and upper motor neuron signs like an upgoing plantar response, due to its role in myelin synthesis and neuronal health.
Tabes Dorsalis: A late manifestation of syphilis, tabes dorsalis can cause a range of neurological symptoms including absent reflexes and upper motor neuron signs. However, it typically presents with lightning pains, ataxia, and other specific symptoms that might not be mentioned here.
Cauda Equina Syndrome: This condition, resulting from compression of the nerve roots in the lumbar spine, can cause a variety of lower extremity symptoms, including reflex changes and potentially upper motor neuron signs if the compression affects the spinal cord. However, it more commonly presents with bowel and bladder dysfunction, and pain.

Spinal Cord Compression: Any condition causing compression of the spinal cord (e.g., tumor, abscess, hematoma) can present with a combination of upper and lower motor neuron signs, depending on the level and extent of compression. This is a medical emergency that requires prompt diagnosis and treatment.
Multiple Sclerosis: An autoimmune disease affecting the central nervous system, multiple sclerosis can cause a wide range of neurological symptoms, including upper motor neuron signs. While less likely to cause absent ankle reflexes specifically, it's a condition that should not be missed due to its significant impact on quality of life and the availability of treatments.

Conus Medullaris Lesion: A lesion affecting the conus medullaris, the lowest part of the spinal cord, can cause a combination of upper and lower motor neuron signs, but it is relatively rare and typically presents with specific symptoms like saddle anesthesia and bowel/bladder dysfunction.
Subacute Combined Degeneration: A condition resulting from Vitamin B12 deficiency, leading to degeneration of the dorsal columns and lateral corticospinal tracts, can cause upper motor neuron signs and reflex changes, but it's less common and usually associated with more widespread neurological deficits.