How would you diagnose a ...

A 47-year-old man was admitted for investigation of worsening tremor and unsteadiness. He had slurring dysarthria, a coarse action tremor in the right hand and an ataxic gait. Retinitis pigmentosa was detected on fundoscopy, as shown in the figure. On questioning, he admitted to night blindness. He had H/o pneumonia and autoimmune haemolytic anaemia, which required a splenectomy. A year later, he developed pneumococcal septic arthritis and low serum concentrations of IgG (1.8 g/l), IgA, and IgM. He was started on IM Ig later changed to IV Ig. Aged 37, he presented with a 3-month H/o memory impairment, poor concentration, and right leg clumsiness. 

The neurological symptoms remained static, but he developed diarrhoea and abdominal pain; a duodenal biopsy confirmed an enteropathy. Investigations at the time of the neurological symptoms in 1998 revealed serum vitamin E concentration was low at 2.75 Amol/l. His serum vitamin A concentration was lower than the reference range. An MRI of the brain revealed an extensive high signal change in the white matter of the frontal and parietal lobes. He showed objective improvement in muscle power and coordination after treating with oral vitamin E supplements at a dose of 80 mg/day. The MRI changes resolved after treatment, as shown in the figure.