**Core Concept**
The 50 g glucose challenge test is a screening test for gestational diabetes mellitus (GDM) in pregnant women. This test measures the body's ability to regulate blood glucose levels after a glucose load.

**Why the Correct Answer is Right**
The cutoff value of plasma glucose in the 50 g glucose challenge test is 7.8 mmol/L (140 mg/dL). If the plasma glucose level is higher than this threshold, it indicates an increased risk of GDM. The test is based on the principle that a high glucose level can lead to fetal macrosomia, birth asphyxia, and other complications. The 50 g glucose challenge test stimulates the pancreas to release insulin, which helps to regulate blood glucose levels.

**Why Each Wrong Option is Incorrect**
**Option A:** This value is lower than the established cutoff, and it would not be an accurate indicator of GDM risk.
**Option B:** This value is higher than the established cutoff and would be an accurate indicator of GDM risk, but it is not the correct answer.
**Option C:** This value is not a valid cutoff for the 50 g glucose challenge test.

**Clinical Pearl / High-Yield Fact**
In clinical practice, the 50 g glucose challenge test is often followed by a 100 g glucose tolerance test (GTT) for women who have a positive result. The GTT is a more definitive test for diagnosing GDM.

**Correct Answer: B. 7.8 mmol/L (140 mg/dL)**