An itchy rash on the inner thighs that is spreading to nearby areas is most commonly due to a fungal infection (ringworm), especially in warm, sweaty regions of the body. The best treatment is to use a proper antifungal cream such as clotrimazole, ketoconazole, or terbinafine applied twice daily for at least 3-4 weeks and continued for 1-2 weeks after the rash clears. Avoid using steroid-mixed creams, as they can worsen the infection and cause it to spread. Keep the area clean and dry, wear loose cotton clothing, change undergarments daily, and avoid sharing towels to prevent reinfection. If the rash is extensive, very itchy, or not improving, a doctor may prescribe oral antifungal medicines for a complete cure.