All-Trans Retinoic Acid (TRA) Is a Metabolite Generated from Vitamin

All-trans retinoic acid (tRA) is a metabolite generated from vitamin A in the diet, and is a key component in generating an appropriate balance of FoxP3+ regulatory CD4 T cells versus T_H17 effector CD4 T cells in mucosal tissues. Patients with some autoimmune diseases, such as psoriasis, derive benefit from oral synthetic retinoid treatment for their condition. This indicates:

A) A possible role for genetic defects in retinol dehydrogenase, an enzyme required to covert vitamin A into tRA, in autoimmunity
B) That T_H17 cells play a role in all autoimmune diseases
C) That tRA likely has systemic actions unrelated to the immune system
D) That environmental factors such as diet can impact the onset of autoimmunity
E) A likely role for tRNA in the development of effector CD4 T cells in the skin