Which amino acids are considered positively charged (basic) at physiological pH?

Lysine, arginine, and histidine are the amino acids that are considered positively charged (basic) at physiological pH, which is typically around 7.4.

Lysine and arginine contain side chains that bear primary and guanidinium amine groups respectively, allowing them to acquire a positive charge at neutral pH due to the protonation of their amine groups. Histidine, while it can be neutral depending on the environment due to its imidazole side chain, has a pKa around 6.0, which means it is often positively charged at physiological pH. This property is crucial for protein function and interactions, particularly in enzyme active sites and binding areas.

In contrast, the other options consist of amino acids that either do not possess a charge at physiological pH or are negatively charged. This makes the choice of lysine, arginine, and histidine the only accurate representation of positively charged amino acids under standard physiological conditions.