STR typing repeatedly fails on an extract prepared from a blood-soaked leather belt, even though adequate human DNA is quantified. After purification through a silica column, amplification succeeds. Which component of the crude extract most likely inhibited the polymerase?

• A Sodium azide used as preservative on the swab
• B Keratin fragments from the leather
• C Haem-derived pigments from lysed erythrocytes ✓
• D Ethidium bromide carried over from staining

Explanation

Haem compounds and other porphyrin-related pigments leached from blood are classic PCR inhibitors of thermostable polymerases, along with indigo from denim and humic acids from soil. They survive co-extraction with DNA and cause complete amplification failure despite good DNA quantification, since quantitation assays do not detect inhibitors. Purification columns separate inhibitors from DNA and restore amplification, which fits the case outcome.

Reference: Butler, Forensic DNA Typing, 2nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.