Recent research has demonstrated that members of some families are unable to feel pain - VCE - SSCE Biology - Question 3 - 2007 - Paper 1

The key information in the question indicates that the affected individuals have normal responses to touch, temperature, and pressure, which suggests that their sensory neurons are functioning properly. If their neurons were unable to respond to external stimuli (option A), we would expect to see abnormal responses to these stimuli. Similarly, options C and D propose more severe dysfunctions that wouldn't align with the observed behavior of these individuals. Therefore, the most reasonable conclusion is that the ion channels are blocked in a specific group of neurons, preventing pain signals from being processed effectively.