Check your hypothesis against the clinical signs:

Tone
Any sign of UMN lesion or hypotonia (cerebellar?) NO
But there is some reduced tone in the left arm.

Power
He is weak and its come on over the past few days: this is classical of GBS: an ascending peripheral motor and sensory poylneuropathy.

Coordination
Normal: as expected

Sensation
Normal: So can it still be GBS???

YES! The sensory signs are often vary vague: there may be only back pain as the presenting feature.

Reflexes
Clinical tip: no reflexes suggests a lower motor neurone problem. Could it me MND? Very unlikely: there’s only LMN signs and the onset of the illness is too acute.

Other things
GBS: measure the Forced Vital Capacity. If this is low: the patient may need ventilation.

Also remember: cardiac conduction deficits (monitor the patient on a cardiac monitor)

Remember FVC monitoring in GBS.

Cannot be Multiple Sclerosis coz you are more likely to see upper motor neurone signs wit hmotor weakness

Stroke causes upper motor neurone features

MND has mixed UMN+LMN signs whereas this man has pure LMN signs

Creutzfeldt Jacob disease would have manifested with myoclonic jerks

Myaesthenia gravis shows worsening function with repeated use of the muscle/muscle group