The vaginal swab is the primary specimen collected in cases of sexual assault because it allows for the recovery of biological evidence such as semen, epithelial cells, and potential DNA from the perpetrator. This sample is crucial for forensic analysis to identify and confirm the assailant through DNA profiling. Timing of collection is essential, typically within 72 hours post-assault, to maximize evidence retrieval. Buccal swabs, blood, and urine samples serve different purposes and are not optimal for detecting assault-related biological material from the victim’s genital tract.

Reference: Knight's Forensic Pathology, Knight B, 4th Edition.