**Core Concept**
Target sign on ultrasound (USG) refers to a characteristic appearance resembling a bull's eye or a target, often observed in conditions where there is a thickened or hypertrophied muscular layer surrounding a central lumen. This sign is particularly useful in diagnosing conditions involving the gastrointestinal tract.

**Why the Correct Answer is Right**
In Congenital Hypertrophic Pyloric Stenosis (CHPS), the target sign is seen on ultrasound due to the thickened pyloric muscle (typically >4 mm) and the narrowing of the pyloric lumen. The hypertrophied muscle forms a thick, hyperechoic ring surrounding the central lumen, which appears as a hypoechoic area. This creates the characteristic target appearance. The pyloric channel length is also typically increased in CHPS. The target sign is a reliable indicator for diagnosing CHPS, especially in infants and young children.

**Why Each Wrong Option is Incorrect**
**Option B:** Gastric ulcers do not typically show a target sign on ultrasound, as they are usually characterized by a focal defect in the gastric mucosa with or without surrounding inflammation, rather than a thickened muscular layer.
**Option C:** Achalasia is a motility disorder of the esophagus, characterized by a failure of the lower esophageal sphincter to relax, and does not typically present with a target sign on ultrasound.
**Option D:** The target sign is not seen in all the above conditions, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
When performing an ultrasound for suspected Congenital Hypertrophic Pyloric Stenosis, it's essential to measure the pyloric channel length and thickness, as well as assess the pyloric muscle thickness, to confirm the diagnosis.

**✓ Correct Answer: A. Congenital hypertrophic pyloric Stenosis**