Q. A well-respected biochemistry researcher, is hosting a seminar on the nutritional aspect of amino acids. He goes into great detail about the different categories of amino acids, explaining that they are not just mere building blocks of proteins but also have significant roles in nutrition and health. One interesting point he raises is about 'essential amino acids,' which, as he describes, are those that the human body cannot synthesize and must therefore be obtained from dietary sources. He challenges his curious medical students, asking, "Can anyone tell me which of the following is an example of an essential amino acid?"