The commonest pathogen responsible for acute pyelonephritis in pregnancy is:
**Core Concept**
Acute pyelonephritis in pregnancy is a common complication of urinary tract infections (UTIs) that affects the kidneys. The pathogenesis involves bacterial invasion of the renal parenchyma, leading to inflammation and potential damage to the kidneys.
**Why the Correct Answer is Right**
The most common pathogen responsible for acute pyelonephritis in pregnancy is Escherichia coli (E. coli). This is due to the fact that E. coli is a common inhabitant of the gastrointestinal tract and can easily contaminate the urinary tract, especially in pregnant women with urinary tract abnormalities or catheterization. The ascending route of infection is the most common mechanism, where bacteria ascend from the vagina and colonize the upper urinary tract.
**Why Each Wrong Option is Incorrect**
**Option A:** Klebsiella pneumoniae is a less common cause of pyelonephritis in pregnancy and typically affects patients with underlying urinary tract abnormalities or catheterization.
**Option B:** Staphylococcus saprophyticus is a common cause of UTIs in young, sexually active women but is less commonly associated with pyelonephritis in pregnancy.
**Option C:** Group B Streptococcus (GBS) is a common cause of UTIs in pregnancy, particularly in the first trimester, but is less commonly associated with pyelonephritis.
**Clinical Pearl / High-Yield Fact**
Pregnant women are more susceptible to UTIs due to the natural compression of the ureters by the expanding uterus, which can lead to vesicoureteral reflux and increased risk of infection.
**Correct Answer: D. Escherichia coli.**