In a case of suspected myasthenia gravis, which diagnostic procedure is least indicated?

In a case of suspected myasthenia gravis, a nitrogen washout test is the least indicated procedure. Myasthenia gravis primarily affects the neuromuscular junction, leading to muscle weakness that is often more prominent with activity and improves with rest. Diagnostic procedures for myasthenia gravis typically focus on assessing muscle function and the presence of antibodies.

The Tensilon challenge is a relevant diagnostic tool for myasthenia gravis, where edrophonium chloride is administered to assess for temporary improvement in muscle strength, confirming the diagnosis. Basic spirometry may be used to evaluate respiratory muscle function in patients who might be experiencing respiratory difficulties due to muscle weakness.

Arterial blood gases can be informative in cases of significant respiratory failure, which can occur in advanced cases of myasthenia gravis. However, the nitrogen washout test is primarily used to assess lung function and ventilation-perfusion mismatching in conditions like pulmonary disorders, making it less applicable in the context of myasthenia gravis. Thus, among the options provided, it is the least indicated for diagnosing or evaluating suspected myasthenia gravis.