**Core Concept**
Significant bacteriuria is a critical criterion for diagnosing urinary tract infections (UTIs). It refers to the presence of a certain number of bacteria per milliliter of urine, which indicates a true infection rather than contamination. The definition of significant bacteriuria is essential for distinguishing between true infections and asymptomatic colonization.

**Why the Correct Answer is Right**
The correct answer is 10^5 CFU (colony-forming units) per milliliter of urine. This is based on the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) criteria, which define significant bacteriuria as 10^5 CFU/mL of urine for women and 10^6 CFU/mL for men. This distinction is necessary due to the higher likelihood of asymptomatic colonization in women. The presence of bacteria in the urine is detected using a urine culture, which involves inoculating the urine sample onto a culture medium and counting the number of colonies that form.

**Why Each Wrong Option is Incorrect**
**Option A:** 10^3 CFU/mL is too low to be considered significant bacteriuria, as it may be due to contamination or asymptomatic colonization.
**Option B:** 10^7 CFU/mL is higher than the recommended threshold for significant bacteriuria, but it is not the correct answer.
**Option C:** 10^2 CFU/mL is even lower than Option A and is unlikely to represent a true UTI.

**Clinical Pearl / High-Yield Fact**
A urine culture should be performed in patients with symptoms suggestive of a UTI, and the results should be interpreted in conjunction with the patient's clinical presentation and risk factors.

**Correct Answer: B. 10^5 CFU/mL**