**Core Concept**
The underlying principle being tested is the management and complications of asymptomatic bacteriuria in pregnancy, specifically the risk of developing pyelonephritis. **Urinary tract infections (UTIs)** are common in pregnancy due to physiological changes.

**Why the Correct Answer is Right**
A first episode of symptomatic bacteriuria in pregnancy significantly increases the risk of developing **pyelonephritis**, a serious infection of the kidney. The correct answer reflects this increased risk, which is approximately 30% if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** This option underestimates the risk.
**Option B:** This option overestimates the risk.
**Option D:** This option is also incorrect as it does not accurately reflect the risk.

**Clinical Pearl / High-Yield Fact**
It's crucial to diagnose and treat **asymptomatic bacteriuria** in pregnancy to prevent complications like **pyelonephritis**.

**Correct Answer:** C. 30%