Okay, let's tackle this question. The patient, Nandini, had an upper respiratory tract infection and then after 15 days developed hematuria. I need to figure out the probable diagnosis here.

First, I remember that post-infectious glomerulonephritis is a common complication after a streptococcal infection, especially a throat infection. The typical time frame is 1-2 weeks after the infection, which matches the 15 days mentioned here. The main symptoms would be hematuria, sometimes with proteinuria, and maybe hypertension or edema. So that's a strong candidate.

Another possibility could be IgA nephropathy, which is associated with infections and can present with hematuria. But the timing here is more typical for post-streptococcal glomerulonephritis. Also, IgA nephropathy often has a more insidious onset.

Hemolytic-uremic syndrome (HUS) is another glomerular disease, but it's more common after E. coli infections, usually from undercooked food, and presents with microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. The timeline here doesn't fit as well.

Goodpasture's syndrome is an autoimmune disease affecting the lungs and kidneys, but it's less likely here without pulmonary symptoms like hemoptysis. Plus, the timing after an infection is not typical for Goodpasture's.

So, the most probable diagnosis here is post-streptococcal glomerulonephritis. The key points are the recent URTI, the time frame, and the presentation of hematuria.

**Core Concept**
Post-infectious glomerulonephritis (PIGN) is a delayed hypersensitivity reaction to **Group A β-hemolytic streptococcus (GAS)**, typically presenting 1–2 weeks after pharyngitis or 3–4 weeks after impetigo. It involves immune complex deposition in glomeruli, leading to hematuria, proteinuria, and renal dysfunction.

**Why the Correct Answer is Right**
Post-streptococcal glomerulonephritis (PSGN) is the most likely diagnosis. Streptococcal antigens form immune complexes with host antibodies, depositing in glomerular basement membranes. This activates the **complement system** and recruits inflammatory cells, causing capillary injury. The 15-day interval aligns with the classic latency period between infection and renal involvement.

**Why Each Wrong Option is Incorrect**
**Option A:** IgA nephropathy (Berger’s disease) is associated with recurrent hematuria after upper respiratory infections but lacks the 1–2 week latency seen in PSGN.
**Option B:** Hemolytic-uremic syndrome (HUS) involves microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury, typically after *E. coli* infection, not streptococcal.
**Option D:** Goodpasture’s disease presents with pulmonary hemorrhage and rapidly progressive glomerulonephritis, not a delayed post-infection course.

**Clinical Pearl / High-Yield Fact**
Remember the **"2-week rule"**: PSGN occurs 1–2 weeks after streptococcal

✓ Correct Answer: D. Post streptococcal glomerulonephritis