Differential Diagnosis for Decreased Sensation in Lower Extremities with Laboratory Findings

• Single most likely diagnosis:
  • D. Cobalamin (Vitamin B12) deficiency: This is the most likely diagnosis due to the presence of macrocytosis (indicated by MCV of 125 fL) and neutrophils with hypersegmented nuclei on the peripheral blood smear, which are classic findings for megaloblastic anemia caused by vitamin B12 deficiency. Vitamin B12 deficiency can also cause neurological symptoms, including decreased sensation in the lower extremities due to demyelination of the dorsal columns of the spinal cord.
• Other Likely diagnoses:
  • Folate deficiency: Although not listed as an option, folate deficiency can also cause macrocytic anemia with similar laboratory findings. However, it typically does not cause the same neurological symptoms as vitamin B12 deficiency.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • None of the options provided (A, B, C) directly correlate with the laboratory findings described, but it's essential to consider other causes of macrocytic anemia and neurological symptoms. Iron (C) deficiency typically causes microcytic anemia, not macrocytic. Vitamin C (A) and Ascorbic acid (B) deficiencies (scurvy) can cause various symptoms but are not directly linked to the specific hematological findings described.
• Rare diagnoses:
  • Other rare causes of macrocytic anemia, such as certain medications or congenital disorders affecting DNA synthesis, could be considered but are less likely given the information provided.