___________________________ is an encyclopedic source of chemical substance data.

The Merck Index is indeed an encyclopedic source of chemical substance data. It provides comprehensive information on a wide range of chemicals, including their properties, structures, and uses. This reference is highly valued in both academic and professional settings, as it serves as a reliable resource for chemists, pharmacists, and other professionals who require detailed chemical information.

Other choices may provide relevant pharmaceutical data but do not serve the same comprehensive purpose as The Merck Index. For instance, the Physicians Desk Reference primarily focuses on drug product information and prescribing details, while the American Drug Index offers a compilation of drug names and associated details but lacks the depth of chemical substance data found in The Merck Index. Drug Facts and Comparison provides comprehensive information on medications but is more oriented toward comparing drug products rather than serving as an encyclopedic resource for chemical compounds. The Merck Index stands out due to its extensive coverage and authoritative status in the field of chemistry.