Which of the following is not an essential amino acid?

First, let us recall the biological definition of an essential amino acid. An essential amino acid is one that the human body cannot synthesize in sufficient quantity, so it must be obtained directly from the diet. Conversely, a non-essential amino acid can be synthesized within the body from other metabolic intermediates.

The universally accepted list of essential amino acids for humans contains nine members, which we may write symbolically inside math delimiters as $$\text{His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val}.$$ Translating these abbreviations, the list is:

Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine.

Now we examine each option given in the problem and compare it with this list.

Option B gives $$\text{Leucine},$$ which we immediately see in the essential list. So Leucine is an essential amino acid.

Option C gives $$\text{Valine},$$ which also appears in the essential list. Therefore Valine is likewise essential.

Option D gives $$\text{Lysine},$$ another name present in the essential list. Hence Lysine is essential as well.

Option A gives $$\text{Tyrosine}.$$ Tyrosine does not appear in the nine-member essential list. The human body can synthesize Tyrosine from the essential amino acid Phenylalanine through the following metabolic transformation:

$$\text{Phenylalanine} \;\xrightarrow[\text{phenylalanine hydroxylase}]{\;\text{O}_2,\;\text{BH}_4\;} \;\text{Tyrosine}.$$

Because this conversion is readily carried out in normal metabolism, Tyrosine does not need to be supplied by the diet and therefore is classified as non-essential.

From the above comparison, only Tyrosine fails the criterion of being essential.

Hence, the correct answer is Option A.