Circle's stock has experienced a significant decline, approaching its initial trading price, as investors voice concerns over the company's profit sustainability and regulatory risks. Despite Circle's Q3 performance surpassing expectations, the company has increased its full-year operating expense forecast, indicating potential challenges for future profit growth. A major portion of Circle's revenue is derived from reserve income, which is under pressure due to decreasing yields on short-term investments. This reliance on a single income stream, coupled with regulatory uncertainties, has contributed to investor skepticism and the recent stock downturn.