Assignment task:
A 23 y/o male, a lifelong vegetarian, presented with 3 months' history of pervasive depressed mood, anhedonia, easy fatigability, poor concentration, guilt, low self-esteem, death wishes, psychomotor retardation, insomnia, anorexia, and third person auditory hallucinations, which were mood congruent. He had four previous episodes of a similar nature over 4 years, which responded to Nardil® (phenelzine) and antipsychotics. He also complained of gait disturbances and stiffness of 8 weeks' duration, which were not present in earlier episodes. The patient met DSM-IV criteria for major depressive disorder, recurrent, currently severe depression with psychotic symptoms. There was no contributory family history. On physical examination, the patient had pallor, knuckle hyperpigmentation, a beefy tongue, generalized weakness, diminished Achilles reflex in both legs, exaggerated patellar reflex in the left leg, spasticity, and loss of vibration sense in both lower limbs, extensor plantar reflexes, ataxic gait, and positive Romberg's sign, with no cognitive deficits. He scored 22 on the Hamilton Depression Rating Scale (0-7 is normal; above 20 is considered clinical depression). His investigations revealed Hb of 11.9 (normal, 13-17) g/dl, hematocrit of 32.9 (normal, 40-50) L/L, MCH of 43.9 (normal, 27-32) pg, MCHC of 36.2 (normal, 315-345) g/L, MCV of 121.1 (normal, 80-101) fl, and on peripheral blood smear macrocytic normochromic anemia and hyper-segmented neutrophils were seen. Vitamin B12 level was 155 (normal, 175-885) pg/ml and folic acid level was 7.2 (normal, 3-17) ng/ml. Other investigations, including MRI of the brain and spine were normal.
What part of the brain would be most affected?