**Core Concept:** DNA karyotyping is a cytogenetic technique used to analyze the chromosomes in cells to identify chromosomal abnormalities, which can be helpful in diagnosing genetic disorders and cancers. The goal is to obtain high-quality chromosome preparations for accurate analysis.

**Why the Correct Answer is Right:**

The correct answer is **C. Osmotic lysis**. Osmotic lysis is a method used to prepare cells for karyotyping by exposing them to a hypotonic solution that swells the cells, causing the cell membrane to rupture and releasing the chromosomes for extraction and staining. This method ensures that the chromosomes are fully extended, making it easier to identify and analyze abnormalities.

**Why Each Wrong Option is Incorrect:**

A. **Chemical fixation (Option A):** Chemical fixation involves treating the cells with fixatives like methanol or acetic acid to preserve the cell structure and prevent degradation. However, this method does not effectively release the chromosomes, making it unsuitable for karyotyping.

B. **Physical disruption (Option B):** Physical disruption methods involve breaking cells physically, but they do not preserve chromosome structure, making it difficult to identify and analyze abnormalities.

D. **Air-drying (Option D):** Air-drying involves allowing cells to dry on a glass slide, which results in dehydrated and clumped chromosomes, making it unsuitable for accurate chromosome analysis.

**Clinical Pearl / High-Yield Fact:**

Chromosomal abnormalities can indicate various conditions, such as Down syndrome, Klinefelter syndrome, and Turner syndrome. Accurate karyotyping requires proper chromosome preparation methods, like osmotic lysis, to ensure clear and extended chromosome structures for effective diagnosis.