Final answer:
Obese individuals and the elderly, even with no comorbidities, are at a higher risk of complications during peripheral nerve blocks due to issues related to nerve localization, drug pharmacokinetics, and age-related physiological changes.
Step-by-step explanation:
Patients at higher risk for complications when undergoing peripheral nerve blocks include b) Obese individuals and d) Elderly with no comorbidities. Obesity can increase the risk due to difficulties in nerve localization, potential for respiratory complications, and challenges with the pharmacokinetics of local anesthetics used during the block. The elderly, even without comorbidities, are at a higher risk because of age-related physiological changes, which can affect the absorption, distribution, and excretion of the anesthetic drugs, as well as increased susceptibility to nerve damage and hemodynamic instability. It is less common for young adults (a) or healthy athletes (c) to be considered high-risk for standard peripheral nerve block procedures unless they have other underlying risk factors.