**Core Concept**
The presence of motile spermatozoa in vaginal secretions is a crucial indicator of recent sexual activity. This concept is related to the evaluation of sexual assault cases and forensic medicine.

**Why the Correct Answer is Right**
The presence of motile spermatozoa in vaginal secretions within 72 hours of intercourse is a reliable indicator of recent sexual activity. This is because sperm can survive for up to 5 days in the female reproductive tract, but their motility decreases rapidly. Motility is a more reliable indicator of recent intercourse than the presence of sperm alone. The motility of sperm is influenced by factors such as the volume of semen deposited, the time elapsed since intercourse, and the presence of seminal stains.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the presence of motile spermatozoa in vaginal secretions can be detected within a shorter timeframe than 72 hours.
**Option B:** This option is incorrect because the presence of non-motile spermatozoa is not a reliable indicator of recent sexual activity.
**Option C:** This option is incorrect because the presence of spermatozoa in vaginal secretions can be due to other factors such as masturbation or medical procedures.

**Clinical Pearl / High-Yield Fact**
In cases of alleged sexual assault, the presence of motile spermatozoa in vaginal secretions within 72 hours of intercourse is strong evidence of recent sexual activity.

**Correct Answer: C. 72 hours**