Which statement best describes the mechanism of fumonisin toxicity with respect to sphingolipid metabolism?

• A. Fumonisins cause renal tubular necrosis.
• B. Fumonisins cause hepatic toxicity by oxidative stress in mitochondria.
• C. Fumonisins cause immunosuppression by T-cell inhibition.
• D. Inhibition of sphinganine to sphingosine in sphingolipid biosynthesis leads to hepatic dysfunction.

Answer: (D)

Fumonisins disrupt sphingolipid metabolism by inhibiting ceramide synthase, the enzyme that converts sphinganine into ceramide. This blocks the progression of sphingolipid biosynthesis, causing buildup of sphinganine and a shortage of ceramide and downstream sphingolipids. The resulting imbalance in sphingolipid homeostasis affects membrane structure and signaling in liver cells, leading to hepatic dysfunction. The statement that describes inhibition of the step from sphinganine toward downstream sphingolipids (toward sphingosine/ceramide) best captures this mechanism, since the key effect is blocking the conversion of sphinganine to downstream sphingolipid products.