A 58 year-old female presents to the clinic with c/o a rash and mild joint pain

D Question 8 A 58 year-old female presents to the clinic with c/o a rash and mild joint pain that started 2 days ago. She states that she felt like she may have had a “low fever”, but didn’t check temperature. She denies any other associated symptoms. She reports no PMH except 5 days ago, she went to urgent care and was diagnosed with UTI. She was prescribed Ciprofloxacin at that time because that is the “antibiotic I took for UTI in the past and it really helped: On exam, you note a maculopapular rash to arms and torso. VS: Temp: 99.8, HR: 84, BP: 118/68, RR: 18. Exam is otherwise benign. You order U/A, C&S to determine if Cipro is sensitive, CBC, and BMP. Meanwhile, the U/A results reveal: White cell casts. The CBC revealed eosinophilia. BMP: Creatinine is elevated. Which of the following is the most likely diagnosis? • Chronic Kidney Disease • Interstitial nephritis • Ureteral stones • Lupus

Based on the clinical presentation and laboratory findings of the 58-year-old female patient, the most likely diagnosis is interstitial nephritis. Here’s the reasoning behind this conclusion:

Symptoms and Timeline: The patient presents with a rash, mild joint pain, and a history of recent UTI treatment with ciprofloxacin. The onset of symptoms began shortly after starting the antibiotic.

Laboratory Findings:

U/A with White Cell Casts: The presence of white cell casts in the urine is indicative of renal parenchymal disease, particularly involving the interstitium, which is consistent with interstitial nephritis.
CBC with Eosinophilia: Eosinophilia is often associated with allergic reactions and drug-induced interstitial nephritis, supporting the diagnosis.
Elevated Creatinine: This suggests a decline in renal function, which is common in interstitial nephritis as the renal tubules are affected.
Differentiating Factors:

Chronic Kidney Disease (CKD): While the elevated creatinine might suggest CKD, the acute nature of her symptoms and the associated rash do not fit the typical presentation of CKD.
Ureteral Stones: Ureteral stones would likely present with more acute renal colic and hematuria, which are not indicated in this case.
Lupus: While lupus can cause renal involvement and may present with a rash, the acute onset of symptoms and the specific lab findings point more towards interstitial nephritis, especially considering the recent antibiotic use.
Given these points, interstitial nephritis is the most likely diagnosis, particularly as it is often drug-induced and fits the clinical picture presented.

References
For further reading on the subject, consider looking at sources like:

National Kidney Foundation (NKF): Information on interstitial nephritis and kidney diseases.
UpToDate: Clinical guidelines and information on acute interstitial nephritis.
Medscape: Detailed discussions about the causes, symptoms, and diagnosis of interstitial nephritis.
You can explore more about this condition in detail through these platforms.