**Core Concept**
The screening for gestational diabetes mellitus (GDM) involves testing for impaired glucose tolerance in pregnant women. This is crucial to prevent potential complications such as macrosomia, preterm labor, and preeclampsia.

**Why the Correct Answer is Right**
A random glucose challenge test (GCT) is the initial screening test for gestational diabetes. This test involves a 50g glucose solution that is administered orally, and the patient's blood glucose level is measured after 1 hour. A level of 140 mg/dL or higher is considered abnormal and warrants further testing with a 100g oral glucose tolerance test (OGTT). The GCT is used because it is non-fasting, easy to administer, and has a high sensitivity for detecting gestational diabetes.

**Why Each Wrong Option is Incorrect**
**Option A:** The oral glucose tolerance test (OGTT) is a more definitive test for diagnosing gestational diabetes, but it is typically performed after an abnormal GCT result.

**Option B:** Fasting blood sugar is used to diagnose diabetes mellitus in non-pregnant individuals, but it is not the preferred screening test for gestational diabetes in pregnant women.

**Option C:** Glycocyalted hemoglobin measurement (HbA1c) is used to monitor long-term glycemic control in diabetic patients, but it is not a suitable screening test for gestational diabetes.

**Clinical Pearl / High-Yield Fact**
The American College of Obstetricians and Gynecologists (ACOG) recommends screening for gestational diabetes between 24 and 28 weeks of gestation in all pregnant women, regardless of risk factors.

**✓ Correct Answer: D. Random glucose challenge**