AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020
Articles
Correct and Preferred Usage
The article a is used before the aspirate h (eg, a historic occasion) and nonvocalic y (eg, a ubiquitous organism). Abbreviations and acronyms are preceded by a or an according to the sound following (eg, a UN resolution, an HMO plan) (see 13.8, Agencies, Organizations, Foundations, Funding Bodies, and Others and 7.0, Grammar).
a hypothesis [h sound] |
an ultraviolet source [u sound] |
a WMA report [d sound] |
a UV source [y sound] |
a hematocrit [h sound] |
an honorarium [o sound] |
an MD degree [e sound] |
an NIH grant [e sound] |
a historic operation [h sound] |
a historical reenactment [h sound] |