A stain recovered from a rape victim's clothing gives a strongly positive acid phosphatase test. The suspect is known to have undergone vasectomy several years earlier. Which statement BEST explains this result?

• A The result indicates contamination of the stain with vaginal secretions, which are rich in acid phosphatase
• B Vasectomy increases the concentration of acid phosphatase because seminal vesicle fluid predominates
• C A positive acid phosphatase test proves that intact spermatozoa were deposited in the stain
• D The test detects prostatic acid phosphatase, which remains present in the ejaculate after vasectomy ✓

Explanation

Acid phosphatase in semen is of prostatic origin, so the test remains positive even when the ejaculate contains no spermatozoa, as after vasectomy or in azoospermia. For this reason it is only a presumptive or screening test and cannot establish the presence of spermatozoa; confirmation rests on demonstration of sperms or of semen-specific antigen such as PSA. Vaginal secretions contain low levels of acid phosphatase, far below the concentrations produced by prostatic fluid.

Reference: Parikh's Textbook of Medical Jurisprudence and Toxicology, 7th ed.

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