While arginine is a highly basic amino acid, it is not technically the "most basic" amino acid. Lysine is generally considered the most basic amino acid due to its pKa value.

Here's why:

* pKa Values: The pKa value is a measure of how readily a molecule donates a proton (H+). A lower pKa indicates a stronger acid, while a higher pKa indicates a stronger base.

* Lysine's pKa: The side chain amino group of lysine has a pKa of around 10.5, making it a relatively strong base.

* Arginine's pKa: The guanidino group of arginine has a pKa of around 12.5. While this is higher than lysine's pKa, it's important to note that the pKa of the guanidino group is influenced by its resonance structures, making it a weaker base than the simple amino group of lysine.

Therefore, lysine's higher pKa and its simpler amino group make it a stronger base than arginine.

However, both arginine and lysine are considered basic amino acids due to the presence of their positively charged side chains at physiological pH.