**Question:** Amino acid with dissociation constant closest to physiological pH is -

A. Lysine
B. Arginine
C. Histidine
D. Aspartic Acid

**Core Concept:**

The dissociation constant (Ka) is a measure of the strength of an acid in solution. In the context of amino acids, a higher Ka value indicates that the acid is more likely to be protonated at physiological pH (around 7.4). Amino acids are considered weak acids because they readily lose a proton (H+) to form a conjugate base in the body.

**Why the Correct Answer is Right:**

Among the given options, Lysine (Ka ≈ 10^5.4) and Arginine (Ka ≈ 10^12) have Ka values closer to the physiological pH (7.4). Amino acids with higher Ka values are better protonated, which is essential for their proper function in the body.

**Why Each Wrong Option is Incorrect:**

Aspartic Acid (Ka ≈ 10^4.3) has a lower Ka value than Lysine and Arginine, indicating that it is less likely to be protonated at physiological pH and may not perform its functions effectively.

**Histidine** (Ka ≈ 10^5.6) has a higher Ka value than Lysine but not as high as Arginine or Aspartic Acid. It is more likely to be protonated, but not as effective as Lysine or Arginine.

**Clinical Pearl:**

Lysine and Arginine are essential amino acids required for protein synthesis, immune system function, and various enzymatic reactions. Lysine is involved in wound healing and immune function, while Arginine plays a crucial role in nitric oxide production, urea synthesis, and immune response modulation. Having a Ka value closer to physiological pH ensures effective protonation and function of these amino acids in the body.

**Correct Answer:** Lysine (Amino Acids with higher Ka values are better protonated at physiological pH, which is essential for their proper function in the body. Lysine has the second highest Ka value among the given options, making it the best choice for a correct answer.