Okay, let's tackle this question step by step. The patient is a 28-year-old G3P2 woman at 35 weeks' gestation presenting with painful uterine contractions and dark, altered vaginal bleeding. On exam, she has a tender uterus, elevated BP (160/100), tachycardia (100 bpm), and P/V shows altered blood with 2 cm dilation. The question is asking for the most likely etiology of her bleeding.

First, I need to recall the common causes of bleeding in the third trimester. The main differentials are placenta previa and placental abruption. Placenta previa typically presents with painless, bright red bleeding, often occurring after 20 weeks. However, in this case, the bleeding is dark and altered, and there's associated pain and contractions, which points more towards placental abruption. Placental abruption is characterized by the separation of the placenta from the uterine wall before delivery, leading to bleeding that's often dark because it's trapped behind the placenta. The presence of uterine tenderness and contractions further supports this, as abruption can cause uterine irritability and pain.

The patient's hypertension (160/100) is also significant. Preeclampsia is a risk factor for placental abruption. Hypertension can lead to placental ischemia, increasing the risk of abruption. The tachycardia might be due to hypovolemia from the bleeding or as a response to pain and stress.

Now, looking at the options (though they aren't listed here), the correct answer is likely placental abruption. Let's go through the other options. Placenta previa would present without pain, so that's a key differentiator. Uterine rupture is another possibility, but it's less common and usually in women with prior uterine surgeries. Trauma or cervical changes could be considered, but the clinical picture fits abruption better. The altered blood suggests it's old blood, which is consistent with a retroplacental clot in abruption.

Clinical pearl: Remember that painless third-trimester bleeding is typically placenta previa, while painful bleeding with uterine tenderness and contractions suggests placental abruption. Also, hypertension and preeclampsia are risk factors for abruption.

**Core Concept**
This scenario tests the differential diagnosis of third-trimester bleeding, focusing on **placental abruption**, characterized by **painful uterine contractions**, **tenderness**, and **dark altered blood** due to retroplacental hemorrhage. **Preeclampsia** (elevated BP) is a key risk factor.

**Why the Correct Answer is Right**
Placental abruption occurs when the placenta partially or completely detaches from the uterine wall before delivery, causing **painful contractions** and **uterine tenderness**. The **dark, altered blood** results from retroplacental clotting. Hypertension (160/100) increases vascular resistance, leading to placental ischemia and abruption risk. The presence of **2 cm dilation** suggests preterm labor as a complication of abruption.

**Why Each Wrong Option is Incorrect

✓ Correct Answer: B. Placenta abruption