Which agency is primarily responsible for enforcing phytosanitary regulations in the United States?

The United States Department of Agriculture (USDA) is the agency primarily responsible for enforcing phytosanitary regulations in the United States. This role is carried out through the Animal and Plant Health Inspection Service (APHIS), which operates under the USDA. APHIS is tasked with protecting the country's agricultural and natural resources from the introduction and spread of pests and diseases, ensuring that imported and exported agricultural products comply with phytosanitary standards.

Phytosanitary regulations are crucial as they aim to prevent the entry of harmful pests and diseases that could negatively affect crops, livestock, and the overall ecosystem. By enforcing these regulations, the USDA helps safeguard the agricultural economy and public health.

The other agencies listed have different mandates and focus areas. For example, the Environmental Protection Agency (EPA) primarily focuses on environmental protection and regulating pollutants, while the Federal Trade Commission (FTC) deals with consumer protection and antitrust issues. The United States Department of Commerce (DOC) focuses on economic growth and job creation and does not enforce phytosanitary regulations. Thus, the USDA's specific role and expertise in agricultural health make it the correct answer for this question.