**Core Concept**
Vitamin B12 deficiency leads to impaired red blood cell production and neurological damage due to disruption of myelin synthesis and axonal integrity, particularly affecting the posterior columns of the spinal cord.

**Why the Correct Answer is Right**
B12 deficiency causes megaloblastic anaemia and subacute combined degeneration of the spinal cord, primarily involving the dorsal and lateral columns. This results in sensory ataxia, loss of proprioception, and bilateral sensory disturbances—classic signs of posterior column dysfunction. The deficiency impairs DNA synthesis in rapidly dividing cells like bone marrow and neurons, leading to demyelination and axonal degeneration. The clinical triad of anaemia, sensory ataxia, and loss of vibration and position sense is highly specific for B12 deficiency.

**Why Each Wrong Option is Incorrect**
Option A: B1 deficiency causes beriberi, characterized by peripheral neuropathy, Wernicke’s encephalopathy, and neurological signs like ataxia and weakness, but not posterior column dysfunction or anaemia. It does not cause megaloblastic anaemia.
Option C: SSPE (Subacute Sclerosing Panencephalitis) is a rare, progressive encephalitis caused by measles virus, presenting with seizures, dementia, and motor dysfunction—no anaemia or posterior column signs.
Option D: Multiple sclerosis causes demyelination but typically presents with relapsing-remitting symptoms, optic neuritis, and motor weakness, not the specific sensory loss or anaemia seen here.

**Clinical Pearl / High-Yield Fact**
In older adults, anaemia with sensory ataxia and loss of vibration sense should always prompt testing for vitamin B12 deficiency—this is the most common cause of such a presentation and a classic exam trap for misdiagnosing MS or other disorders.

✓ Correct Answer: B. B12-deficit