**Core Concept**
Pyelonephritis is a type of urinary tract infection (UTI) that affects the kidneys. In pregnancy, the risk of pyelonephritis increases due to physiological changes in the urinary tract, such as ureteral dilation and decreased peristalsis. The most common cause of pyelonephritis in pregnancy is a bacterium that typically colonizes the gastrointestinal tract.

**Why the Correct Answer is Right**
The most common cause of pyelonephritis in pregnancy is Escherichia coli (E. coli), a gram-negative bacterium. E. coli can ascend from the lower urinary tract to the kidneys through the ureters, causing pyelonephritis. This ascent is facilitated by the physiological changes in the urinary tract during pregnancy, such as ureteral dilation and decreased peristalsis, which allow bacteria to colonize the upper urinary tract. The virulence factors of E. coli, such as adhesins and pili, enable it to attach to and colonize the uroepithelial cells, leading to infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** Other gram-negative bacteria, such as Klebsiella pneumoniae and Proteus mirabilis, can also cause pyelonephritis in pregnancy, but they are less common than E. coli.
* **Option B:** Gram-positive bacteria, such as Staphylococcus saprophyticus, can cause UTIs, but they are less common causes of pyelonephritis in pregnancy.
* **Option C:** Fungal infections, such as candiduria, can occur in pregnancy, but they are rare causes of pyelonephritis.

**Clinical Pearl / High-Yield Fact**
The American College of Obstetricians and Gynecologists (ACOG) recommends screening for asymptomatic bacteriuria in pregnant women at 12-16 weeks and 24-28 weeks of gestation, and treating any positive results to prevent pyelonephritis.

**Correct Answer: D. E. coli. Escherichia coli.**