**Core Concept**
Enteric fever, caused by Salmonella Typhi (S. Typhi) and Salmonella Paratyphi, is a systemic infection that affects the gastrointestinal and hematological systems. Diagnosis in the second week of illness is crucial for guiding appropriate treatment and preventing complications.

**Why the Correct Answer is Right**
In the second week of enteric fever, the diagnosis is best made by blood culture. This is because the bacteremia, which is the presence of bacteria in the blood, is more pronounced during this period. Blood cultures are a sensitive and specific method for detecting S. Typhi and S. Paratyphi. The bacteria are more likely to be isolated from the blood during the second week due to the bacteremia-induced immune response, making blood culture a reliable diagnostic tool.

**Why Each Wrong Option is Incorrect**
**Option A:** Widal test is a serological test used to detect the presence of antibodies against S. Typhi and S. Paratyphi. However, it is not the most reliable method for diagnosing enteric fever in the second week, as the antibodies may not have reached detectable levels yet.

**Option B:** Stool culture can be used to diagnose enteric fever, but it is less sensitive than blood culture, especially in the second week of illness. Stool cultures are more useful for detecting the carrier state.

**Option C:** Imaging studies like ultrasound or CT scans may show complications of enteric fever, such as splenic or liver abscesses, but they are not a primary method for diagnosing the infection.

**Clinical Pearl / High-Yield Fact**
The Widal test is not a reliable diagnostic tool for enteric fever in the second week of illness, as the antibodies may not have reached detectable levels yet. This is a classic exam trap, and students should be aware of the limitations of serological tests in diagnosing enteric fever.

**Correct Answer:** B. Stool culture can be used to diagnose enteric fever, but it is less sensitive than blood culture, especially in the second week of illness.