**Question:** Earliest (first) L.A. used clinically

A. **L-aspartate**
B. **L-glutamate**
C. **D-aspartate**
D. **D-glutamate**

**Core Concept:** L-amino acids are the essential components of proteins and peptides in humans, while D-amino acids are rare in humans but more prevalent in certain bacteria, fungi, and viruses.

**Why the Correct Answer is Right:** The correct answer, L-glutamate (Option B), is the first L-amino acid used clinically because it is the most abundant amino acid in the central nervous system and plays a crucial role in neurotransmission. L-aspartate (Option A) is also a neurotransmitter involved in ionotropic glutamate receptors, but it is not as prevalent as glutamate. D-aspartate (Option C) is rare in humans but more common in certain bacteria, fungi, and viruses. Lastly, D-glutamate (Option D) is also rare in humans but more prevalent in certain bacteria, fungi, and viruses, making it less likely to be the earliest amino acid used clinically.

**Why Each Wrong Option is Incorrect:**

A. L-aspartate (Option A) is not the earliest amino acid used clinically because it is not as prevalent as glutamate in the central nervous system and glutamate has a more significant role in neurotransmission.

B. L-glutamate (Option B) is the correct answer because it is the most abundant amino acid in the central nervous system and plays a crucial role in neurotransmission.

C. D-aspartate (Option C) is not the earliest amino acid used clinically due to its rarity in humans and its prevalence in certain microorganisms.

D. D-glutamate (Option D) is not the earliest amino acid used clinically because it is less prevalent in humans compared to glutamate and more prevalent in certain microorganisms.

**Clinical Pearls:**

1. L-glutamate is the most abundant amino acid in the central nervous system, playing a vital role in the excitatory neurotransmission.
2. The understanding of L-glutamate's role in neurotransmission and its implications in neurological disorders like glutamate toxicity and its treatments has led to its clinical use.
3. The rarity of D-amino acids in humans and their prevalence in certain microorganisms implies that they are less likely to be the first amino acids used clinically.
4. Glutamate receptor antagonists, such as N-methyl-D-aspartate (NMDA) receptor antagonists, are used in treating neurological disorders like stroke, traumatic brain injury, and multiple sclerosis.

**Correct Answer Explanation:**

The correct answer is L-glutamate (Option B), as it is the most abundant amino acid in the central nervous system, readily available, and plays a crucial role in excitatory neurotransmission. Understanding its role in neurological disorders like glutamate toxicity and treatments has led to its clinical use.