**Core Concept**
The one-hour glucose challenge test (GCT) is used to screen for gestational diabetes mellitus (GDM) in pregnant women. The test measures glucose levels one hour after consuming a 50g glucose solution. A normal value indicates low risk for GDM, while an abnormal value requires further evaluation with a standard glucose tolerance test (OGTT).

**Why the Correct Answer is Right**
If the one-hour GCT value is ≥ 140 mg/dL, it indicates an increased risk for GDM. The standard OGTT is recommended to further evaluate glucose tolerance and diagnose GDM. The OGTT involves measuring glucose levels at fasting, one hour, two hours, and sometimes three hours after consuming a 100g glucose solution. This test provides a more comprehensive assessment of glucose metabolism and helps diagnose GDM.

**Why Each Wrong Option is Incorrect**
**Option A:** Values < 140 mg/dL are considered normal, and a standard OGTT is not required. **Option B:** Values between 140-199 mg/dL may indicate impaired glucose regulation, but a standard OGTT is not always necessary. **Option C:** This option is not a valid value for the one-hour GCT. **Clinical Pearl / High-Yield Fact** Remember that the one-hour GCT is a screening test, and an abnormal value does not confirm GDM. A standard OGTT is necessary for diagnosis and further evaluation. **Correct Answer: D. 140 mg/dL**