A 50-year-old male presents with a 3-month history of weakness, cramping, fasciculations, and loss of muscle mass in his left arm. The most likely diagnosis is

• A. Amyotrophic lateral sclerosis.
• B. Guillain-Barré syndrome.
• C. Parkinson's disease.
• D. Myasthenia gravis.

Answer: (A)

Progressive weakness with cramping and noticeable muscle fasciculations and atrophy points to a motor neuron disease affecting both upper and lower motor neurons. In this condition, lower motor neuron loss produces fasciculations and muscle wasting, while upper motor neuron involvement leads to later spasticity and hyperreflexia as the disease advances. It often starts in a single limb and then spreads to other regions, which fits a subacute, focal onset in the arm that progresses over months. Guillain-Barré syndrome tends to be an acute, ascending, symmetric weakness with areflexia and sometimes sensory symptoms, not a focal, slowly progressive arm problem. Parkinson’s disease centers on tremor, rigidity, and bradykinesia rather than focal limb wasting and fasciculations. Myasthenia gravis causes fluctuating weakness that worsens with use and often involves ocular muscles, without marked muscle wasting or fasciculations. Therefore, the most likely diagnosis is amyotrophic lateral sclerosis.