**Core Concept**
Pregnancy-induced hypertension (PIH), also known as gestational hypertension, is a condition characterized by high blood pressure in pregnant women, typically after 20 weeks of gestation. The pathophysiology involves vascular endothelial dysfunction, increased vascular resistance, and activation of the renin-angiotensin-aldosterone system.

**Why the Correct Answer is Right**
The earliest sign of PIH is often subtle and nonspecific, but it is typically the **mild elevation of blood pressure**. This is usually a systolic blood pressure of 140 mmHg or higher, or a diastolic blood pressure of 90 mmHg or higher on two occasions at least 4 hours apart. The increase in blood pressure is often accompanied by proteinuria, which is the presence of excess proteins in the urine.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not specified, so we'll assume it's a distractor. However, if it's a specific sign like headaches or visual disturbances, these can be symptoms of PIH but are not the earliest signs.
* **Option B:** This option is also not specified, but if it's a sign like edema or urinary frequency, these can be symptoms of PIH but are not the earliest signs.
* **Option C:** This option is not specified, but if it's a sign like abdominal pain or vaginal bleeding, these can be symptoms of PIH but are not the earliest signs.

**Clinical Pearl / High-Yield Fact**
The American College of Obstetricians and Gynecologists (ACOG) recommends that women with high blood pressure during pregnancy be monitored closely for signs of PIH, such as severe hypertension, proteinuria, and fetal growth restriction. Early detection and management of PIH can help prevent complications in both mother and fetus.

**Correct Answer: A. Mild elevation of blood pressure**