**Core Concept**
In the first week of typhoid fever, the bacterium Salmonella Typhi is primarily found in the bloodstream, making blood culture the most reliable method for early diagnosis. This is because the bacteria multiply rapidly in the bloodstream during the initial stages of the disease.

**Why the Correct Answer is Right**
Blood culture is the investigation of choice in the first week of typhoid fever because the bacteria are more likely to be present in the blood during this period. This is due to the bacterium's ability to invade and multiply within the endothelial cells lining the blood vessels, leading to bacteremia. The Widal test, which detects antibodies against Salmonella Typhi, may not be positive until the second week of the disease, making it less reliable in the first week. Stool culture and urine culture can also be used to diagnose typhoid, but they are less sensitive than blood culture in the first week.

**Option A:** Blood culture is the most sensitive method for detecting Salmonella Typhi in the first week.
**Option B:** Widal test is the most common method for diagnosing typhoid fever, but it is not the best choice in the first week.
**Option C:** Stool culture can be used to diagnose typhoid, but it is less sensitive than blood culture in the first week.
**Option D:** Urine culture is not typically used for diagnosing typhoid fever in the first week.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the sensitivity of blood culture decreases after the first week of typhoid fever, making stool culture and urine culture more reliable methods for diagnosis in later stages of the disease.

**✓ Correct Answer: A. Blood culture**