Which test, when positive, can be used for the presumptive identification of Candida albicans?

The germ tube test is the preferred method for the presumptive identification of Candida albicans. When a yeast is incubated in serum or a similar liquid medium at 37°C, the formation of germ tubes can be observed, which are elongated extensions from the yeast cells. This characteristic is particularly associated with Candida albicans and assists in its recognition.

Other tests mentioned, such as the API 20C, provide identification of a broader range of yeasts and may not specifically highlight Candida albicans. The bile solubility test is generally used for differentiating streptococci rather than fungi and the oxidase test is mainly for identifying certain bacteria based on their ability to oxidize specific substrates. Thus, the germ tube test stands out as a crucial and specific tool in recognizing Candida albicans due to its distinctive morphological features.