Total Melanonychia of 20 Nails as a Rare Manifestation of Vitamin B12 Deficiency

A 47-year-old black male presented with total melanonychia of all 20 nails, a rare manifestation of vitamin B12 deficiency, alongside symptoms like asthenia, hyporexia, and significant weight loss. Laboratory tests revealed severe anemia and extremely low vitamin B12 levels (<30 pg/mL). Treatment with high doses of cyanocobalamin, pyridoxine, and thiamine led to significant improvement in nail discoloration and constitutional symptoms after 3 months, with vitamin B12 levels rising to 940 pg/mL. This case highlighted the potential for vitamin B12 deficiency to cause diffuse nail hyperpigmentation, a condition typically associated with other systemic causes.