**Question:** DNA analysis can be done from all except

A. Blood cells
B. Skin cells
C. Saliva
D. Urine

**Correct Answer:** D. Urine

**Core Concept:** DNA analysis involves extracting and studying the genetic material (DNA) from cells to identify individuals, detect genetic disorders, or trace back criminal activities. DNA can be obtained from various sources, including blood cells, skin cells, saliva, and urine.

**Why the Correct Answer is Right:** DNA can be extracted from blood cells (RBCs, leukocytes) and skin cells (keratinocytes) as they contain high amounts of DNA. Saliva is also a suitable source of DNA, particularly for oral genetic testing, as it contains various types of cells (e.g., leukocytes, epithelial cells) with intact DNA. However, urine lacks the necessary cells and cellular components required for accurate DNA extraction and analysis.

**Why Each Wrong Option is Incorrect:**

**A. Blood cells:** DNA can be extracted from blood cells, making this a wrong answer choice. Blood cells contain high amounts of DNA, making them an appropriate source for genetic testing.

**B. Skin cells:** DNA can be extracted from skin cells, particularly keratinocytes, which are the predominant cell type in the epidermis. This choice is also incorrect as it supports the correct answer (C).

**C. Saliva:** Saliva is a suitable source of DNA, particularly for oral genetic testing. Saliva contains various types of cells (e.g., leukocytes, epithelial cells) with intact DNA, making it a valid option.

**D. Urine:** Urine lacks the necessary cells and cellular components required for accurate DNA extraction and analysis. Urine primarily contains filtered waste products and does not contain the required cells and genetic material for DNA analysis.

**Clinical Pearl:** DNA analysis from blood and saliva is commonly used in forensic science, paternity testing, and genetic research. Urine and skin samples are not suitable for DNA analysis due to the absence of sufficient cells and genetic material.