Okay, let's tackle this question. The user provided a question about a hypertensive young lady with a normal DTPA scan and a small left kidney on ultrasound. The next investigation is to be determined. The correct answer is missing, but I need to figure out what it should be.

First, the core concept here is likely related to renal artery stenosis, especially since the patient is young and hypertensive. Renal artery stenosis can lead to renovascular hypertension. The DTPA scan is normal, which assesses renal function and drainage. However, the ultrasound shows a small left kidney, which might indicate chronic ischemia due to stenosis.

Next, the correct answer should be the investigation that confirms renal artery stenosis. Common investigations include Doppler ultrasound, CT angiography, MR angiography, or renal artery Doppler. Since the patient already had an ultrasound showing a small kidney, the next step would be to check for renal artery stenosis. Doppler ultrasound of the renal arteries is a non-invasive first step. If that's inconclusive, CT or MR angiography might be used. But given the options, Doppler ultrasound is probably the next step.

The incorrect options might include things like urine tests, blood tests, or other imaging that don't directly assess the renal arteries. For example, a CT scan of the abdomen without contrast or a renal biopsy would be less relevant here. Also, since the DTPA scan is normal, it's not about acute obstruction but rather chronic issues like stenosis.

Clinical pearl: Renal artery stenosis in young hypertensive patients, especially with asymmetric kidney size, should prompt evaluation of renovascular hypertension. The key is to check for renal artery stenosis using Doppler ultrasound or angiography.

Putting it all together, the next investigation after a small kidney on USG and normal DTPA would be Doppler ultrasound of the renal arteries to assess for stenosis. So the correct answer is Doppler ultrasound of renal arteries.

**Core Concept**
The clinical scenario suggests renovascular hypertension, commonly caused by renal artery stenosis. A small kidney on ultrasound implies chronic ischemia, while a normal DTPA scan rules out acute obstruction. Renal artery stenosis in young patients is often fibromuscular dysplasia (FMD), necessitating vascular imaging.

**Why the Correct Answer is Right**
The next step is **Doppler ultrasound of the renal arteries** to assess for stenosis. Fibromuscular dysplasia (FMD) is a leading cause of renovascular hypertension in young women, characterized by abnormal vascular wall development. Doppler ultrasound is non-invasive, cost-effective, and detects turbulent blood flow in stenotic arteries. If equivocal, cross-sectional imaging (CT/MR angiography) may follow.

**Why Each Wrong Option is Incorrect**
**Option A:** Urine analysis (e.g., for proteinuria) is not specific for renal artery stenosis.
**Option B:** Serum creatinine/eGFR staging is standard but does not address renovascular causes.
**Option C:** Intravenous pyelogram (IVP) is outdated and insensitive for vascular lesions.
**Option D:** Renal biopsy is contraindicated without evidence of intrinsic parenchymal disease.

**Clinical Pearl**
Never assume small kidney size on

✓ Correct Answer: C. Digital subtraction angiography