**Question:** A 3-year-old boy presents with fever; dysuria and gross hematuria. Physical examination shows a prominent suprapubic area which is dull on pecussion. Urinalysis reveals red blood cells but no proteinuria. Which of the following is the most likely diagnosis?

A. Urinary tract infection (UTI)
B. Hydronephrosis
C. Nephrotic syndrome
D. Tubulointerstitial nephritis

**Correct Answer:**

**Core Concept:** Urinary tract infections (UTIs) are a group of infections that affect different parts of the urinary system, including the kidneys, ureters, bladder, and urethra. These infections are common in children and can lead to various clinical manifestations, such as fever, dysuria, and hematuria.

**Why the Correct Answer is Right:** The prominent suprapubic area that is dull on percussion is a clinical sign of hydronephrosis, which is a condition characterized by the accumulation of urine in the kidney due to obstruction or dysfunction of the urinary tract. Hydronephrosis can be caused by UTIs, leading to the obstruction of urine flow and causing the clinical features seen in the patient's presentation.

**Why Each Wrong Option is Incorrect:**

**A. Urinary tract infection (UTI):** While UTIs can cause fever, dysuria, and hematuria, they usually do not lead to hydronephrosis. Instead, UTIs usually cause renal parenchymal involvement, which can lead to acute pyelonephritis, renal scarring, or tubulointerstitial nephritis.

**B. Hydronephrosis:** This option is correct, as it is a potential complication of UTIs. However, option A is more specific to UTIs, while this option addresses the broader consequence of UTIs.

**C. Nephrotic syndrome:** Nephrotic syndrome is a group of clinical syndromes characterized by severe proteinuria (>3.5 g/24 h), hypoalbuminemia, hyperlipidemia, and edema. It is unrelated to the presented symptoms and signs in the question.

**D. Tubulointerstitial nephritis:** Tubulointerstitial nephritis is an inflammation of the renal tubules and interstitium, often caused by infections or drugs. It can lead to renal dysfunction but is not directly related to the described symptoms and signs.

**Clinical Pearl:**

In children, UTIs are usually caused by bacteria that ascend the urethra to reach the bladder due to immature immune system development, underdeveloped urethral sphincter, and a high anus-genital distance. The clinical presentation in this case is due to hydronephrosis resulting from the obstruction caused by the ascending bacteria. The kidneys will not be directly affected.

Please note that the correct answer is B (hydronephrosis), as the symptoms and signs presented in the question are directly related to this condition, which results from the obstruction to urine flow, leading to increased intrarenal pressure and renal dysfunction. The other options are not directly related