## Core Concept
The question tests knowledge of nucleic acid composition, specifically the types of bases found in DNA and RNA. Nucleic acids are composed of nucleotides, each containing a phosphate group, a sugar molecule (deoxyribose in DNA and ribose in RNA), and one of four nitrogenous bases.

## Why the Correct Answer is Right
Uracil is a base found in RNA, replacing thymine which is found in DNA. The standard bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T). In RNA, adenine (A), guanine (G), cytosine (C), and uracil (U) are the standard bases.

## Why Each Wrong Option is Incorrect
* **Option A:** Adenine is a purine base found in both DNA and RNA, making it incorrect as an answer to this question.
* **Option B:** Guanine is another purine base present in both DNA and RNA, so it's not the correct answer.
* **Option C:** Cytosine is a pyrimidine base found in both DNA and RNA, which means it's also not the correct answer.

## Clinical Pearl / High-Yield Fact
A key point to remember is the difference between DNA and RNA bases: DNA contains **A**, **G**, **C**, and **T**, while RNA contains **A**, **G**, **C**, and **U**. This difference is crucial for various molecular biology techniques and understanding genetic material.

## Correct Answer Line
**Correct Answer: D. Uracil**