**Core Concept**
The patient has ingested a caustic substance (lye), which has caused severe esophageal and mediastinal damage, leading to a mediastinitis and subsequent complications.

**Why the Correct Answer is Right**
The ingestion of lye causes severe alkaline burns to the esophagus and surrounding tissues, leading to a necrotizing esophagitis and mediastinitis. The subcutaneous crepitus on palpation is a result of gas formation due to the bacterial fermentation of necrotic tissue. The widening of the mediastinum on chest x-ray is indicative of the mediastinitis, which is an infection of the tissues in the mediastinum. The pleural effusion is a result of the inflammation and infection spreading to the pleural space.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the complications of lye ingestion. The patient's symptoms and signs are not typical of a simple esophageal stricture.
**Option B:** This option is incorrect because it is more typical of a pulmonary embolism, which would not cause subcutaneous crepitus or a pleural effusion.
**Option C:** This option is incorrect because it does not accurately describe the complications of lye ingestion. The patient's symptoms and signs are not typical of a simple gastroesophageal reflux disease (GERD).

**Clinical Pearl / High-Yield Fact**
Lye ingestion is a medical emergency that requires prompt treatment, including endoscopy and esophageal dilation, as well as broad-spectrum antibiotics to treat the mediastinitis.

**Correct Answer: D. Esophageal rupture and subsequent mediastinitis.**