What is a common complication associated with untreated diphtheria?

Heart failure is a common complication associated with untreated diphtheria due to the effects of the toxin produced by the Corynebacterium diphtheriae bacteria. This toxin can lead to myocarditis, an inflammation of the heart muscle, which impairs the heart's ability to pump effectively. As the condition progresses, it can result in heart failure, leading to symptoms such as shortness of breath, fatigue, and fluid retention.

While pneumonia can occur in diphtheria, especially if there is respiratory involvement, it is not the most characteristic or direct complication of the disease. Kidney damage can also occur due to systemic effects of infections or toxins, but it is less commonly associated with diphtheria specifically. Septic shock is typically related to more severe, widespread infections, while diphtheria's primary complications are largely tied to the local effects of the throat infection and the systemic effects of the diphtheria toxin. Therefore, heart failure stands out as a significant potential complication of untreated diphtheria.