**Core Concept**
In obstetrics, the mean gestational sac diameter (MGSD) is a critical parameter used to assess the viability of an early pregnancy. A gestational sac without an embryonic pole on transvaginal ultrasound (TVS) raises concerns about fetal development. The American College of Obstetricians and Gynecologists (ACOG) recommends specific cutoff values for MGSD to diagnose anembryonic gestation.

**Why the Correct Answer is Right**
The correct answer is C. 25mm because, according to ACOG, a mean gestational sac diameter of 25mm or greater on TVS without an embryonic pole is considered anembryonic gestation. This is because a sac of this size is expected to contain an embryo with a detectable pole by this gestational age. The absence of an embryonic pole at this stage suggests a failed pregnancy. The embryonic pole is typically visible on TVS when the gestational sac reaches 16-17mm in diameter.

**Why Each Wrong Option is Incorrect**
**Option A:** A mean gestational sac diameter of 15mm is too small to diagnose anembryonic gestation, as an embryonic pole may not be visible at this stage. **Option B:** A diameter of 20mm is also not sufficient to diagnose anembryonic gestation, as some pregnancies may still be viable at this stage. **Option D:** A diameter of 30mm is indeed anembryonic gestation, but it is not the smallest size at which this diagnosis can be made.

**Clinical Pearl / High-Yield Fact**
When evaluating early pregnancy on TVS, remember that a mean gestational sac diameter of 16-17mm is typically the earliest age at which an embryonic pole should be visible. If no pole is seen at this stage, further evaluation is warranted to assess fetal viability.

**✓ Correct Answer: C. 25mm**