Questions: In a Citrate Test, a "positive result" is indicated by a black line, a light yellow color, a bright green color, a bright blue color.

Transcript text: In a Citrate Test, a "positive result" is indicated by a black line, a light yellow color, a bright green color, a bright blue color.

Solution

The answer is the fourth one: a bright blue color.

Explanation for each option:

A black line: This is not associated with a positive result in a Citrate Test. It might be confused with other tests, such as the SIM test for sulfur reduction.
A light yellow color: This is not indicative of a positive result in a Citrate Test. Yellow color changes are typically associated with acid production in other types of biochemical tests.
A bright green color: This is not the correct indicator for a positive Citrate Test. The medium in a Citrate Test starts as green, and a color change to blue indicates a positive result.
A bright blue color: This is the correct indicator of a positive result in a Citrate Test. The medium contains bromothymol blue, which turns blue when the pH increases due to the utilization of citrate by the organism.

Summary: In a Citrate Test, a "positive result" is indicated by a bright blue color.

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