**Core Concept**
Epidemic dropsy is a condition caused by the consumption of mustard oil contaminated with the seed oil of the Mexican prickly poppy (Argemone mexicana), which contains sanguinarine and dihydrosanguinarine. These alkaloids cause endothelial damage and lead to the characteristic symptoms of the disease.

**Why the Correct Answer is Right**
The first and commonest clinical manifestation of epidemic dropsy is peripheral edema, particularly in the legs and feet. This is due to the accumulation of fluid in the interstitial spaces, which is a result of the endothelial damage caused by the alkaloids. The edema is bilateral and symmetrical, and it is often accompanied by weight gain, which can be significant in some cases.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because while respiratory symptoms can occur in epidemic dropsy, they are not the first and commonest clinical manifestation.

**Option B:**
This option is incorrect because gastrointestinal symptoms can occur in epidemic dropsy, but they are not the primary presentation.

**Option C:**
This option is incorrect because while cardiovascular symptoms can occur in epidemic dropsy, they are not the first and commonest clinical manifestation.

**Clinical Pearl / High-Yield Fact**
Peripheral edema is a hallmark of epidemic dropsy, and it is often the first symptom to appear. This can be a key differentiator from other conditions that cause peripheral edema, such as heart failure or liver disease.

**Correct Answer: B. Gastrointestinal symptoms.**