Okay, let's tackle this question about blighted ovum. The question is asking when a blighted ovum is diagnosed on ultrasound based on the gestational sac. First, I need to recall what a blighted ovum is. From what I remember, it's when the gestational sac forms but there's no embryo inside. So the sac is present, but the embryo didn't develop.

Now, the options aren't given here, but the correct answer is probably related to the size of the gestational sac and the absence of a yolk sac or embryo. Let me think about the typical ultrasound criteria. I think that if the gestational sac diameter is a certain size, like around 18-20 mm, and there's no yolk sac or embryo, that's when they diagnose a blighted ovum. Also, maybe the absence of a fetal pole or cardiac activity after a certain gestational age.

Wait, another point: the crown-rump length (CRL) is measured when the embryo is visible. If the sac is a certain size and there's no embryo, that's a sign. For example, if the sac is 25 mm and there's no yolk sac, or 30 mm with no embryo. Let me check my notes. Oh right, the key is the absence of an embryo when the gestational sac reaches a certain size. I think the standard is when the mean sac diameter is ≥25 mm and there's no yolk sac, or ≥30 mm with no embryo. Alternatively, if the sac is ≥20 mm and there's no yolk sac, or ≥25 mm with no embryo. Different sources might have slightly different numbers, but the principle is the same: the presence of a gestational sac without an embryo or yolk sac beyond a certain size indicates a blighted ovum.

So the correct answer would be when the gestational sac measures a specific size without any embryonic development. The incorrect options might include sizes where the embryo is still expected to be too small to see, or might mention the presence of a yolk sac, which would be a different diagnosis. For example, an anembryonic pregnancy (blighted ovum) is diagnosed when there's no embryo in a gestational sac that's a certain size. The other options might refer to other conditions like a normal early pregnancy where the embryo is just not yet visible, or maybe a non-viable pregnancy due to other reasons.

I need to make sure the explanation covers the key points: the size thresholds for the gestational sac, the absence of yolk sac or embryo, and how that differentiates it from other conditions. Also, the clinical pearl should highlight the size criteria and the importance of follow-up ultrasounds to confirm the diagnosis, as sometimes early pregnancies might show delayed development but still be viable.

**Core Concept**
A blighted ovum (anembryonic pregnancy) is diagnosed on ultrasound when a gestational sac is present but lacks an embryo or yolk sac beyond specific size thresholds. This reflects arrested embryonic development, with the sac continuing to grow but no viable pregnancy.

**Why the Correct Answer is Right**
Diagnosis requires the absence of a yolk sac when the mean sac diameter (MSD) exceeds **25 mm** or absence of an embryo when MSD exceeds **30 mm**. Alternatively, if the

✓ Correct Answer: C. 3 cm