Sunday, February 8, 2009

81 - NIMS MEDICINE february 2009 paper - 6

6q: positive benedict's test is shown by ?

1. glucose
2. homogenitisic acid
3. ascorbic acid
4. ketone bodies

a. 1 , 2 and 3 are correct
b. 1 and 3 are correct
c. 2 and 4 correct
d. only 4 is correct

answer: a . 1 , 2 and 3 are correct .

Urinary sugars when boiled in Benedict's reagent reduce copper sulphate to a reddish cuprous oxide precipitate in hot alkaline medium, the intensity of which is proportional to the amount of sugar present in the urine. The results are reported as I+,2+, etc. depending upon the colour and intensity of the cuprous oxide precipitate.

Benedict's reagent is used as a test for the presence of all monosaccharides, and generally also reducing sugars. These include glucose, galactose, mannose, lactose and maltose. Even more generally, Benedict's test will detect the presence of aldehydes (except aromatic ones), and alpha-hydroxy-ketones, including those that occur in certain ketoses. Thus, although the ketose fructose is not strictly a reducing sugar, it is an alpha-hydroxy-ketone, and gives a positive test because it is converted to the aldoses glucose and mannose by the base in the reagent.

False positive reactions are known to occur due to the presence of non- carbohydrate substances like ascorbic acid, homogentisic acid, creatinine and uric acid. Reducing sugars like lactose, galactose, fructose and pentoses will also give a positive reaction.

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