To determine which amino acid among the given options contains only one basic functional group, let's first understand the structures and properties of each amino acid:
Arginine: Arginine is an amino acid that has a side chain containing a guanidinium group, which is basic. Therefore, Arginine has more than one basic functional group.
Lysine: Lysine has a side chain containing an additional amino group, making it basic. Therefore, it also contains more than one basic group.
Histidine: Histidine contains an imidazole group in its side chain, which acts as a base. Hence, it has more than one basic functional group.
Isoleucine: Isoleucine is one of the simplest amino acids with a side chain that does not contain any basic groups — it only has the primary amino group that all amino acids naturally have. Hence, it contains only one basic functional group.
Conclusion: From the analysis, Isoleucine is the amino acid that naturally occurs with only one basic functional group, which is its primary amino group.
This clarifies why the correct answer is Isoleucine.
