This bill aims to bolster the financial stability of the Federal Hospital Insurance Trust Fund, which supports Medicare Part A. It achieves this by amending the Social Security Act to direct all revenues generated from the net investment income tax (NIIT) into this crucial trust fund, rather than the general fund. Furthermore, the legislation significantly modifies the application of the net investment income tax under the Internal Revenue Code for high-income individuals. For those with modified adjusted gross income exceeding thresholds like $400,000 for single filers or $500,000 for joint filers, the NIIT will apply to a broader base, specifically the greater of their "specified net income" or traditional net investment income. This "specified net income" expands the tax's reach to include certain income derived from a trade or business that was previously exempt, effectively closing loopholes for high earners. The bill also clarifies and expands the types of income subject to the NIIT. It explicitly includes certain foreign income, such as Subpart F income, Global Intangible Low-Taxed Income (GILTI), and Passive Foreign Investment Company (PFIC) income, within the tax base. Additionally, it specifies that net operating losses (NOLs) cannot reduce income subject to the NIIT and directs the Treasury Secretary to issue guidance on the treatment of previously taxed income distributions. These amendments are designed to ensure that more investment and certain business income from high-income individuals contributes to Medicare's solvency. The provisions of this Act are slated to take effect for taxable years beginning after December 31, 2025.