What is the most likely diagnosis for a pregnant patient at 30 weeks of gestation presenting with severe right flank pain radiating to the groin, rigors, chills, and numerous pus cells in the urine?

Diagnosis: Acute Pyelonephritis

The most likely diagnosis is B. Pyelonephritis, based on the classic triad of flank pain radiating to the groin, systemic symptoms (rigors and chills indicating fever), and pyuria on urinalysis in a pregnant patient. 1, 2

Clinical Reasoning

Classic Presentation of Pyelonephritis

• Flank pain is nearly universal in acute pyelonephritis and its absence should raise suspicion of an alternative diagnosis 3
• The radiation to the groin is consistent with ureteral involvement in upper urinary tract infection 1
• Rigors and chills indicate fever (≥38°C), which is a hallmark systemic symptom of pyelonephritis 2, 4
• Numerous pus cells (pyuria) on urinalysis confirms urinary tract inflammation and is a key diagnostic finding 2, 4

Why Other Options Are Less Likely

Appendicitis (Option A):

• While appendicitis can occur in pregnancy and may present with right-sided pain, it typically causes right lower quadrant pain rather than flank pain 5
• Appendicitis does not explain the pyuria on urinalysis 5
• The absence of gastrointestinal symptoms and presence of urinary findings make this diagnosis unlikely

Meckel's Diverticulum (Option C):

• This typically presents with painless rectal bleeding or intestinal obstruction, not flank pain 5
• Would not cause pyuria or systemic symptoms of infection in this pattern
• Extremely unlikely in a 30-week pregnant patient with this presentation

Round Ligament Torsion (Option D):

• Round ligament pain in pregnancy is typically sharp, brief, and located in the lower abdomen or groin 5
• Does not cause fever, rigors, or pyuria 5
• Would not explain the systemic inflammatory response

Diagnostic Confirmation

Essential Laboratory Testing

• Urine culture with antimicrobial susceptibility testing should be performed in all cases of suspected pyelonephritis to guide targeted therapy 1, 2, 4
• Urinalysis showing pyuria and/or bacteriuria confirms the diagnosis when combined with compatible clinical presentation 2, 4

Imaging Considerations in Pregnancy

• Initial imaging is not indicated for uncomplicated acute pyelonephritis, even in pregnancy 1, 2
• Imaging should only be performed if the patient remains febrile after 72 hours of appropriate antibiotic therapy or if clinical deterioration occurs 1, 2
• If imaging becomes necessary, ultrasound is preferred initially to evaluate for obstruction or stones, given the pregnancy 1

Critical Management Points for Pregnant Patients

Pregnant patients with pyelonephritis are at significantly elevated risk of severe complications and should be admitted and treated initially with parenteral therapy 4

Why Pregnancy Changes Management

• Pregnancy is classified as a complicated UTI due to increased risk of complications 6
• Risk of preterm labor, sepsis, and maternal-fetal complications is substantially higher 4
• Hospitalization with intravenous antibiotics is mandatory for pregnant patients with pyelonephritis 4

Initial Treatment Approach

• Ceftriaxone 1-2g daily IV is an appropriate first-line parenteral agent for hospitalized patients with pyelonephritis 1
• Alternative parenteral options include cefotaxime 2g three times daily or cefepime 1-2g twice daily 1
• Treatment duration is typically 7-14 days 1

Common Pitfalls to Avoid

• Do not delay treatment while awaiting culture results - begin empiric antibiotics immediately after obtaining urine culture 4, 3
• Do not attempt outpatient management in pregnant patients - pregnancy mandates hospitalization regardless of apparent clinical stability 4
• Do not obtain imaging initially - 95% of patients with uncomplicated pyelonephritis become afebrile within 48 hours of appropriate therapy 1, 2
• Reassess at 48-72 hours - if fever persists beyond 72 hours despite appropriate antibiotics, imaging is then indicated to rule out obstruction, abscess, or stones 1, 2