Which diagnostic tool detects infected hairs by fluorescing yellow-green under ultraviolet illumination?

• A. KOH Mount
• B. Sabouraud Agar
• C. Gram Stain
• D. Wood's lamp

Answer: (D)

Wood's lamp uses ultraviolet light to reveal fungal infections that produce fluorescent compounds. Hairs infected with Microsporum species often glow a yellow-green color under this UV illumination, making the lamp a quick way to localize infected hairs before further testing. The other tools serve different roles: a microscopic view after dissolving keratin shows fungal elements directly; a culture medium grows fungi for identification; a Gram stain highlights bacteria and some fungi but doesn’t rely on fluorescent signals. Because fluorescence isn’t universal to all fungi, a positive yellow-green glow strongly supports a Microsporum infection, while a negative result doesn’t completely rule it out.