Pyelonephritis is an infection of which structure?

• A. Ureters
• B. Kidneys
• C. Bladder
• D. Urethra

Answer: (B)

Pyelonephritis is an infection of the kidneys—the upper urinary tract, involving the renal pelvis and renal parenchyma. This condition arises when bacteria ascend from the bladder or reach the kidneys via the bloodstream. Distinguishing it from infections of the lower urinary tract helps: the bladder (cystitis) and the urethra (urethritis) involve different sites and often present with more localized urinary symptoms and less systemic illness. Clinically, acute pyelonephritis tends to cause fever, flank or costovertebral angle tenderness, and may be accompanied by nausea or vomiting. Lab clues include increased white blood cells, pyuria, and bacteriuria. Treatment is antibiotics, with the route (oral vs IV) and duration guided by severity and patient factors.