Guthrie’s bacterial inhibition test detects:
## **Core Concept**
Guthrie's bacterial inhibition test is a microbiological assay used to detect the presence of certain amino acids, particularly **phenylalanine**, in the blood. This test is specifically designed for the early detection of **phenylketonuria (PKU)**, a genetic disorder characterized by the inability to metabolize phenylalanine.
## **Why the Correct Answer is Right**
The Guthrie test works on the principle that **Bacillus subtilis**, a bacterium used in the test, is inhibited by the presence of phenylalanine. When blood from a patient is applied to a culture medium containing this bacterium, if phenylalanine levels are elevated (as in PKU), the growth of **Bacillus subtilis** is inhibited. This inhibition is directly proportional to the concentration of phenylalanine in the blood. Therefore, the test is a simple yet effective method for screening newborns for PKU.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because Guthrie's test specifically targets phenylalanine, not other amino acids.
- **Option B:** This option is incorrect as the test is not primarily used for detecting galactosemia, which is a different metabolic disorder related to galactose metabolism.
- **Option C:** This option is incorrect because while glucose levels are crucial for diagnosing diabetes and other metabolic conditions, Guthrie's test is focused on amino acid metabolism, specifically phenylalanine.
- **Option D:** This option is incorrect as the test is not used for detecting tyrosinemia, another metabolic disorder but one that involves the metabolism of tyrosine, not phenylalanine directly.
## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that **early detection of PKU** through Guthrie's test allows for dietary intervention (a low-phenylalanine diet) that can significantly improve outcomes and prevent intellectual disability associated with untreated PKU. This test is typically performed on **heel prick blood samples** collected from newborns.
## **Correct Answer:** B. Phenylalanine.