Okay, let's tackle this question. The patient is a 24-year-old G2P1 woman at 39 weeks with painful contractions and dark vaginal bleeding mixed with mucus. The question is asking about the most likely etiology of her bleeding.

First, I need to recall the common causes of bleeding in late pregnancy. The key differentials would be placenta previa and placental abruption. But wait, the patient is also experiencing painful contractions. Placenta previa typically presents with painless, bright red bleeding, right? So if there's pain, that's more likely to be placental abruption. Also, the presence of contractions suggests active labor, which could be associated with abruption. Another possibility is rupture of membranes, but that usually presents with fluid leakage, not blood. Then there's the show, which is the mucus plug mixed with blood, indicating the cervix is softening and effacing, which is a sign of impending labor. The dark blood and mucus might be the show, which is a normal part of labor onset. So maybe the answer is related to the show. Let me check the options again. The correct answer here is probably the show, as it's dark blood mixed with mucus and associated with contractions. Placenta previa is painless, abruption is with pain but usually bright red blood. The show is a normal finding in labor. So the etiology here is the passage of the show, indicating labor is starting.

**Core Concept**
The passage of "show" (mucus plug mixed with blood) is a normal physiologic event marking cervical changes at the onset of labor. It results from the separation of the cervical mucus plug during effacement, often accompanied by dark red blood due to decidual vessel rupture.

**Why the Correct Answer is Right**
The patient’s presentation of dark vaginal bleeding with mucus and painful contractions aligns with the passage of the mucus plug ("show"), a hallmark of impending labor. The dark blood reflects oxidized blood from decidual vessels disrupted during cervical softening and effacement. This is distinct from pathologic bleeding (e.g., placental abruption), which typically presents with bright red blood and absent contractions. The presence of regular contractions further supports the labor-related etiology.

**Why Each Wrong Option is Incorrect**
**Option A:** *Placenta previa* causes painless, bright red bleeding due to placental separation from the lower uterine segment. Pain is absent.
**Option B:** *Placental abruption* presents with bright red bleeding, abdominal pain, and uterine tenderness, but contractions are absent unless superimposed on preeclampsia.
**Option D:** *Ruptured membranes* involve fluid leakage, not blood, and lack the mucus plug component described.

**Clinical Pearl / High-Yield Fact**
"Show" is a normal labor sign: dark blood with mucus indicates cervical changes. Differentiate from abruption (pain + bleeding + no contractions) and previa (painless bright red bleeding). Remember the mnemonic: **"Show = Show up to labor"** (dark blood + mucus + contractions).

**Correct Answer: C. Passage of the mucus plug (show)**

✓ Correct Answer: B. Bloody show