Which of the following is the precursor for adrenaline and thyroxine synthesis?
**Core Concept**
The question tests the understanding of **amino acid** precursors in the synthesis of crucial hormones. Specifically, it involves the biosynthesis pathways of **catecholamines** (like adrenaline) and **thyroid hormones** (like thyroxine). Tyrosine is an essential **amino acid** that serves as a precursor for these hormones.
**Why the Correct Answer is Right**
Tyrosine is the precursor for both adrenaline and thyroxine synthesis. In the adrenal glands, tyrosine is converted into **L-DOPA** by the enzyme **tyrosine hydroxylase**, which then undergoes several transformations to produce adrenaline. For thyroxine synthesis in the thyroid gland, tyrosine residues in **thyroglobulin** are iodinated and coupled to form thyroxine.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not represent tyrosine or any known precursor in the synthesis of adrenaline and thyroxine.
**Option B:** Similarly, this option is incorrect as it is not related to the biosynthesis of these hormones.
**Option D:** This choice is also incorrect because it does not correspond to the amino acid tyrosine.
**Clinical Pearl / High-Yield Fact**
Remembering that **tyrosine** is the common precursor for both adrenaline (through the catecholamine pathway) and thyroxine (in thyroid hormone synthesis) is crucial. This highlights the importance of tyrosine in endocrine function and metabolism.
**Correct Answer:** C. Tyrosine.