**Core Concept**
Hay's sulpher test is a chemical test used to detect the presence of bile salts in urine. Bile salts are produced in the liver and stored in the gallbladder, where they are released into the small intestine to aid in fat digestion. The test relies on the principle that bile salts can reduce sulfuric acid to hydrogen sulfide gas, which is responsible for the characteristic odor of the test.

**Why the Correct Answer is Right**
The Hay's sulpher test involves adding a solution of sodium nitroprusside and sulfuric acid to a sample of urine. If bile salts are present, they will reduce the sulfuric acid to hydrogen sulfide gas, which reacts with the sodium nitroprusside to produce a brown ring. This reaction is specific to bile salts and is not produced by other substances present in urine. The presence of the brown ring indicates a positive result for the test.

**Why Each Wrong Option is Incorrect**
**Option B:** Reducing sugar in urine is detected using Benedict's test, which involves boiling the urine sample with Benedict's reagent. The presence of reducing sugars causes the reagent to change color, but this reaction is different from the one involved in the Hay's sulpher test.
**Option C:** Ketone bodies in urine are detected using Rothera's test, which involves adding a solution of sodium nitroprusside and acetone to the urine sample. The presence of ketone bodies causes the solution to turn pink, but this reaction is not related to the Hay's sulpher test.
**Option D:** Urobilinogen in urine is detected using Ehrlich's test, which involves adding a solution of p-dimethylaminobenzaldehyde to the urine sample. The presence of urobilinogen causes the solution to turn green, but this reaction is different from the one involved in the Hay's sulpher test.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Hay's sulpher test is specific for detecting bile salts in urine, and a positive result can indicate liver disease, bile duct obstruction, or other conditions that affect bile production. A negative result does not rule out these conditions, and further testing may be necessary to confirm the diagnosis.

**✓ Correct Answer: A. Bile salts in urine**