Final answer:
The infant's diaper rash, characterized by a diffuse erythematous macular rash sparing the inguinal folds and without satellite lesions, is likely caused by irritant contact dermatitis from the diaper.
Step-by-step explanation:
The condition described in the question, a diffusely erythematous (non-beet-red) macular rash in the diaper area that spares the inguinal folds and has no satellite lesions, is most often indicative of irritation from the diaper itself or the moisture associated with it, known as diaper dermatitis. Unlike certain infections such as candidiasis, which would typically present with satellite lesions, the lack of these additional symptoms and the presentation suggest that the rash is due to simple skin irritation from the wet or infrequently changed diaper. Hence, the infant's diaper rash is most likely caused by irritant contact dermatitis from the diaper.