**Core Concept**
Protein purification and separation involve various techniques to isolate and separate proteins based on their physical and chemical properties. This process is crucial in biochemistry and molecular biology to obtain high-quality proteins for research, diagnostics, and therapeutic applications.

**Why the Correct Answer is Right**
Protein purification and separation techniques include chromatography (size exclusion, affinity, ion exchange), centrifugation, electrophoresis (SDS-PAGE, Western blot), and dialysis. These methods exploit differences in protein properties such as size, charge, hydrophobicity, and affinity for specific ligands.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because crystallization is not a primary method for protein purification and separation. While it is used in structural biology to obtain high-resolution protein structures, it is not a general technique for protein purification.

**Option B:** This option is incorrect because ultrafiltration is indeed a method used in protein purification and separation. It involves the use of semipermeable membranes to separate molecules based on their size.

**Option C:** This option is incorrect because gel filtration chromatography is a widely used technique for protein purification and separation. It separates proteins based on their size and hydrodynamic radius.

**Option D:** This option is incorrect because density gradient centrifugation is a valid method for protein purification and separation. It separates proteins based on their density and buoyant density.

**Clinical Pearl / High-Yield Fact**
A key principle in protein purification and separation is to exploit differences in protein properties, such as size, charge, and hydrophobicity, to achieve efficient separation and purification.

**Correct Answer: A**