Human insulin differs from beef insulin by how many number of amino acid residues:

**Core Concept**
Human insulin is a protein hormone composed of 51 amino acid residues. It is synthesized by the pancreas and plays a crucial role in glucose homeostasis. The structure and function of human insulin are essential for understanding diabetes mellitus and its treatment.

**Why the Correct Answer is Right**
Human insulin differs from beef insulin by a single amino acid residue, specifically at position B29. Beef insulin has a lysine residue at this position, whereas human insulin has a proline residue. This difference affects the immunogenicity and metabolic activity of the insulin molecule. The substitution of lysine with proline results in a more compact and stable molecule that is less likely to be recognized as foreign by the immune system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it implies a difference in the number of amino acid residues between human and beef insulin, which is not the case.
**Option B:** This option is incorrect because it suggests a difference in the number of amino acid residues between human and beef insulin, which is not true.
**Option C:** This option is incorrect because it implies a difference in the number of amino acid residues between human and beef insulin, which is not the correct answer.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that human insulin is derived from recombinant DNA technology, which allows for the production of large quantities of highly purified insulin with minimal immunogenicity.

**Correct Answer:** A. 1.