**Core Concept**
Asymptomatic bacteriuria (ASB) in pregnancy increases the risk of developing pyelonephritis, a serious urinary tract infection that can lead to significant maternal and fetal morbidity. The risk is particularly high due to the physiological changes of pregnancy, which include ureteral dilatation and decreased bladder capacity.

**Why the Correct Answer is Right**
The risk of pyelonephritis in a pregnant woman with ASB is significantly higher due to the anatomical and physiological changes of pregnancy. The ureters are dilated, and the bladder capacity is decreased, making it easier for bacteria to ascend from the bladder to the kidneys. Additionally, the hormonal changes of pregnancy can lead to relaxation of the ureteral sphincter, further increasing the risk of bacterial ascent. The risk of pyelonephritis is estimated to be around 1-3% in non-pregnant women with ASB, but it is significantly higher in pregnant women, with some studies suggesting a risk as high as 20-30%.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the risk of pyelonephritis in pregnant women with ASB is not significantly increased in the first trimester. The risk is highest in the second and third trimesters due to the anatomical and physiological changes of pregnancy.
**Option B:** This option is incorrect because the presence of symptoms does not necessarily indicate pyelonephritis. Many women with pyelonephritis may be asymptomatic or have non-specific symptoms, making it essential to screen for ASB in pregnant women.
**Option C:** This option is incorrect because the risk of pyelonephritis is not directly related to the presence of a urinary tract anomaly. While certain anomalies may increase the risk of ASB, the risk of pyelonephritis is more closely related to the presence of ASB and the physiological changes of pregnancy.

**Clinical Pearl / High-Yield Fact**
Pregnant women with ASB should be treated with antibiotics to prevent pyelonephritis. Screening for ASB should be performed at the first prenatal visit, and at 25-30 weeks and 35-37 weeks of gestation in women with a history of ASB or urinary tract anomalies.

**Correct Answer:** D.