Final answer:
Given the symptoms of fever, flank pain, irritative vomiting, tachycardia, and signs of infection in urinalysis, the most likely diagnosis is pyelonephritis, a severe kidney infection requiring antibiotic treatment.
Step-by-step explanation:
The patient's symptoms and laboratory findings most likely indicate a diagnosis of pyelonephritis, which is a serious kidney infection.
The presentation of fever/chills, flank pain, nausea and vomiting (N/V/D), tachycardia, and costovertebral angle (CVA) tenderness, along with a complete blood count (CBC) showing leukocytosis suggest a systemic infection. Urinalysis (UA) that reveals pyuria, bacteriuria, hematuria, and white blood cell (WBC) casts further supports the diagnosis of a kidney infection. Pyelonephritis often stems from a lower urinary tract infection that ascends to the kidneys, and the pathogen responsible is commonly a fecal bacterium like Escherichia coli.
Signs and symptoms such as back pain, fever, nausea/vomiting, and potential gross hematuria make pyelonephritis the most likely diagnosis over similar conditions like cystitis or glomerulonephritis. Left untreated, it may lead to serious complications, including sepsis, and should be managed with appropriate antibiotics like fluoroquinolones or cephalosporins.