Which of the Following Is the First Step in the Biosynthetic

Multiple Choice

Which of the following is the first step in the biosynthetic pathway for thyroid hormones that is inhibited by propylthiouracil?

A) Iodide (I-) pump
B) $\mathrm{I}^{-} \rightarrow \mathrm{I}_{2}$
C) $\mathrm{I}_{2}+$ tyrosine
D) Diiodotyrosine (DIT) + DIT
E) Thyroxine $\left(\mathrm{T}_{4}\right) \rightarrow$ triiodothyronine $\left(\mathrm{T}_{3}\right)$