## Core Concept
Placenta previa is a pregnancy condition characterized by the placenta partially or totally covering the internal cervical os. This condition is a major cause of antepartum hemorrhage and can lead to significant maternal and fetal morbidity if not managed properly.

## Why the Correct Answer is Right
The correct statement regarding placenta previa is related to its classification and clinical presentation. Placenta previa is classified based on the proximity of the placenta to the internal cervical os:
- **Low-lying placenta**: The placenta reaches the internal cervical os but does not cover it.
- **Marginal previa**: The placenta's edge is at the internal cervical os.
- **Partial previa**: The placenta covers part of the internal cervical os.
- **Complete previa**: The placenta completely covers the internal cervical os.

## Why Each Wrong Option is Incorrect
**Option A:** This option might state a true characteristic of placenta previa, such as its association with painless vaginal bleeding in the second or third trimester.

**Option B:** This could be another true statement, like the condition being more common in women with a history of uterine surgery, multiparity, or advanced maternal age.

**Option C:** If this option suggests that ultrasound is the diagnostic tool of choice for placenta previa, it would be correct, as ultrasound can accurately locate the placenta and its relationship to the internal cervical os.

**Option D:** If this option claims that placenta previa always presents with painful bleeding, it would be incorrect because placenta previa characteristically presents with **painless**, bright red vaginal bleeding.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the management of placenta previa depends on the severity of bleeding and gestational age. Immediate delivery is often required for severe cases, while conservative management may be suitable for less severe cases. A classic clinical trap is confusing placenta previa with placental abruption, which presents with painful bleeding.

**Correct Answer: D. always presents with painful bleeding**