**Core Concept**
Isoleucine is a non-polar, aliphatic amino acid with a hydrophobic side chain. Homologous substitution refers to the replacement of one amino acid with another that has a similar chemical structure and properties.

**Why the Correct Answer is Right**
Leucine is a non-polar, aliphatic amino acid that is similar in size and hydrophobicity to isoleucine. It has a similar side chain structure and can be substituted for isoleucine in a protein sequence without significantly altering the protein's structure or function. This is because leucine and isoleucine are both branched-chain amino acids, which allows them to fit into similar binding sites on enzymes and other proteins.

**Why Each Wrong Option is Incorrect**
* **Option A:** Valine is also a non-polar, aliphatic amino acid, but it has a slightly smaller side chain than isoleucine and leucine. While it could potentially substitute for isoleucine in some cases, it is not as similar in size and hydrophobicity.
* **Option B:** Phenylalanine is a non-polar, aromatic amino acid that is quite different from isoleucine in terms of its chemical structure and properties. It would not be a good substitute for isoleucine in most protein sequences.
* **Option D:** Threonine is a polar, hydroxyl-containing amino acid that is quite different from isoleucine in terms of its chemical structure and properties. It would not be a good substitute for isoleucine in most protein sequences.

**Clinical Pearl / High-Yield Fact**
When substituting one amino acid for another in a protein sequence, it is essential to consider the chemical properties of the amino acids involved, including their charge, size, and hydrophobicity. This can help predict how the substitution will affect the protein's structure and function.

**Correct Answer:** C. Leucine.