Which of the following instruments should a medical assistant have available for a patient's general physical examination?

For a patient's general physical examination, the otoscope is particularly essential as it enables the healthcare provider to visualize the ear canal and eardrum. This instrument is used to assess ear health, which is a vital part of a comprehensive physical examination. By inspecting the ears, a provider can identify conditions such as infections, blockages, or other abnormalities.

While the other instruments listed also play important roles in physical examinations, their specific utility may vary depending on the circumstances. A stethoscope is primarily used for auscultation to listen to heartbeats and lung sounds, a sphygmomanometer measures blood pressure, and a thermometer assesses body temperature. However, in the context of a general physical examination, the otoscope is particularly crucial for evaluating ear health, making it the most appropriate choice in this instance.