## Core Concept
The patient's presentation of progressive dysphagia, initially to solids and now to liquids as well, is highly suggestive of a **malignant esophageal obstruction**, most likely **esophageal cancer**. The key principle here is that any patient presenting with progressive dysphagia, especially when it advances to include liquids, warrants an urgent investigation to rule out malignancy.

## Why the Correct Answer is Right
The correct answer, **Endoscopy with biopsy**, is the gold standard investigation for diagnosing esophageal cancer. This procedure allows for direct visualization of the esophageal mucosa, identification of any masses or lesions, and collection of tissue samples for histopathological examination. Given the patient's symptoms and progression, an endoscopy with biopsy is crucial for confirming the diagnosis and guiding further management.

## Why Each Wrong Option is Incorrect
- **Option A: Barium swallow** - While a barium swallow can provide information on the anatomy and motility of the esophagus and may show a stricture or mass effect, it does not allow for tissue sampling. It's useful but not definitive for diagnosis.
- **Option B: CT Thorax** - A CT scan of the thorax can help in staging if a malignancy is diagnosed but does not provide a tissue diagnosis. It's not the first line for diagnosis.
- **Option D: Endoscopic ultrasound (EUS)** - EUS is valuable for assessing the depth of tumor invasion and involvement of nearby structures but is typically performed after a tissue diagnosis has been made.

## Clinical Pearl / High-Yield Fact
A key point to remember is the **"red flag" symptoms** for esophageal cancer, which include progressive dysphagia (initially to solids then to liquids), significant weight loss, and regurgitation of food. Patients with these symptoms should undergo **urgent endoscopy** to rule out malignancy.

## Correct Answer Line
**Correct Answer: . Endoscopy with biopsy**