A boy presents with a red rash on the upper thigh with scaling margins and KOH scraping shows hyphae. What is the likely diagnosis?

The clinical presentation described—the red rash on the upper thigh with scaling margins, coupled with KOH scraping that reveals hyphae—strongly suggests a diagnosis of a fungal infection, specifically one caused by Trichophyton rubrum.

In dermatophyte infections, such as those caused by Trichophyton rubrum, the KOH scrape is particularly useful for identifying fungal elements. The presence of hyphae confirms a dermatophyte infection, as these fungi invade the keratinized tissues, leading to the scaling and erythema observed in this case.

This type of rash is often annular or ring-like, which may not be explicitly stated but is common in such dermatophyte infections. The scaling margins can also help differentiate it from other conditions that present with similar symptoms.

By aligning the patient’s symptoms with the findings from the KOH scraping, the diagnosis of a Trichophyton rubrum infection becomes the most plausible conclusion.