Question

The following conditions may present with both upper and lower motor neuron involvement EXCEPT

• A. Subacute combined degeneration of spinal cord
• B. Friedreich's ataxia
• C. Amyotrophic lateral sclerosis
• D. Becker's Dystrophy

Hint:

Three of these damage the nervous system itself; one is a disease of muscle fiber alone.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
We need to find which of these four conditions does NOT present with a mixture of upper and lower motor neuron signs together.

Step 2: Key Formula or Approach:
Combined upper and lower motor neuron involvement happens in diseases that damage both the corticospinal tract and the anterior horn cells or peripheral nerves at the same time, whereas a disease of the muscle fiber itself does not touch the motor neurons at all.

Step 3: Detailed Explanation:
Subacute combined degeneration of the spinal cord, from vitamin B12 deficiency, damages both the corticospinal tracts (upper motor neuron, causing spasticity and extensor plantar responses) and the peripheral nerves (lower motor neuron, causing absent ankle reflexes), giving a true combined picture.
Friedreich's ataxia damages the corticospinal tracts along with the dorsal root ganglia and peripheral sensory nerves, producing extensor plantar responses (upper motor neuron) together with absent deep tendon reflexes (lower motor neuron), another true combined presentation.
Amyotrophic lateral sclerosis directly and simultaneously destroys both the upper motor neurons of the corticospinal tract and the lower motor neurons of the anterior horn cells, making combined upper and lower motor neuron signs its defining hallmark.
Becker's muscular dystrophy, unlike the other three, is a primary disease of the muscle fiber itself, caused by a partially functioning dystrophin protein. It does not damage the motor neurons or corticospinal tracts at all, so it presents purely with lower motor neuron-type weakness and wasting from muscle disease, without any true upper motor neuron involvement.

Step 4: Final Answer:
So the condition that does NOT present with combined upper and lower motor neuron involvement is Becker's Dystrophy, since it is a pure muscle disease.