Excessive supplementation of which fat-soluble vitamin is most likely to cause systemic toxicity (hypervitaminosis)?

Vitamin A (retinol) is the fat-soluble vitamin most classically associated with toxicity when taken in excess. It accumulates in the liver and adipose tissue. Acute toxicity: nausea, vomiting, headache, increased intracranial pressure. Chronic toxicity: bone pain and fractures, hepatotoxicity (steatosis, fibrosis, cirrhosis), dry/rough skin, alopecia, teratogenicity. Vitamin A has the narrowest therapeutic index among fat-soluble vitamins. Vitamin D toxicity (hypercalcemia) can also occur but is less frequently cited. Water-soluble vitamins (B12, C) are generally non-toxic because excess is renally excreted. Among the fat-soluble vitamins (A, D, E, K), Vitamin A has the most documented toxicity. Bone changes (hyperostosis) and liver damage are the hallmarks of chronic Vitamin A toxicity and are high-yield for the INBDE.