Okay, let's tackle this question. The patient is a 56-year-old man with poorly controlled hypertension presenting with sudden left-sided weakness. The key here is to determine the initial investigation needed.

First, I need to consider the differential diagnosis. Sudden onset of focal neurological deficits like left-sided weakness is a red flag for stroke. Since he has hypertension, which is a major risk factor for both ischemic and hemorrhagic strokes, the first step is to rule out an acute stroke.

The core concept here is the urgent need to differentiate between ischemic stroke and intracranial hemorrhage. The initial investigation of choice in such cases is a non-contrast head CT scan. This is because CT is fast, widely available, and can quickly identify hemorrhage, which is critical because the treatment for ischemic stroke (like thrombolytics) is contraindicated in the presence of bleeding.

Now, looking at the options. The correct answer should be a non-contrast CT scan. Other options might include MRI, which is more sensitive for ischemia but takes longer and isn't always immediately available. Lumbar puncture is not typically the first step here unless there's suspicion of meningitis or subarachnoid hemorrhage not seen on CT. Blood tests are important but don't provide immediate imaging of the brain.

So, the wrong options are likely other imaging modalities or tests that don't address the acute need. For example, MRI is more accurate for ischemia but slower. Lumbar puncture is for differentials like meningitis. Blood tests are supportive but not diagnostic. The key is that CT is the first step in acute stroke workup.

**Core Concept**
Sudden focal neurological deficits in a hypertensive patient suggest acute stroke. The priority is to differentiate between **ischemic stroke** (treated with thrombolytics) and **hemorrhagic stroke** (contraindicated for thrombolytics). *Non-contrast head CT* is the **first-line investigation** to identify intracranial hemorrhage.

**Why the Correct Answer is Right**
A **non-contrast computed tomography (CT) scan of the head** is the initial imaging modality for suspected stroke. It rapidly detects hemorrhage, mass effect, or early signs of ischemia (e.g., loss of gray-white differentiation, sulcal effacement). Hypertension is a major risk factor for both stroke subtypes, and immediate imaging is critical to guide treatment (e.g., thrombolysis within 4.5 hours of ischemic stroke onset). CT is preferred over MRI in acute settings due to speed and availability.

**Why Each Wrong Option is Incorrect**
**Option A:** *MRI of the brain* is highly sensitive for ischemia but is slower, less accessible in emergencies, and not superior to CT for detecting acute hemorrhage. **Option B:** *Lumbar puncture* is indicated for meningitis or subarachnoid hemorrhage (if CT is negative), but not as a first-line test. **Option C:** *ECG* or *blood tests* (e.g., CBC, electrolytes) are supportive but do not diagnose stroke directly. **Option D:** *Ultrasound* (e.g., carotid Doppler) is not the initial investigation for acute focal deficits.

**Clinical Pearl / High-Yield Fact**
"**CT first

✓ Correct Answer: B. CT head