**Core Concept:** A codon is a triplet of nucleotides in the genetic code that specifies an amino acid during protein synthesis. They are fundamental units in the process of translation in cells.

**Why the Correct Answer is Right:** A codon is correctly defined as consisting of three nucleotides (Adenine, Uracil, Guanine, and Cytosine) in the genetic code that dictate the sequence of amino acids incorporated into a protein chain. These nucleotides are arranged in a specific order, allowing for the translation of genetic information into functional proteins.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because a codon is composed of three nucleotides, not four.
B. Although ribosomes are involved in protein synthesis, the definition of a codon focuses on the nucleotides, not the ribosome structure or function.
C. This option is incorrect because the focus is on the nucleotides composing a codon, not the ribosomes or other cellular components involved in translation.
D. This option is incorrect because the definition of a codon pertains to the nucleotides involved, not the energy sources or mechanisms of translation.

**Clinical Pearl / High-Yield Fact:** Understanding the genetic code and codons is essential for medical students and professionals, as it underpins the molecular basis of heredity and protein synthesis in cells. This knowledge is vital for interpreting genetic disorders, diagnosing genetic diseases, and understanding the molecular mechanisms in genetics and molecular biology.

**Correct Answer:** A. This is the correct answer because a codon is composed of three nucleotides, specifically adenine (A), uracil (U), guanine (G), and cytosine (C). These nucleotides form the basis of genetic information translation into functional proteins.