Human physiology — Which cells are classically described as dependent primarily on blood glucose as their immediate energy source under normal conditions?

Introduction:
The brain has a high, continuous energy demand and limited fuel reserves. This question probes the traditional teaching that, in the fed and early-fasted states, the central nervous system relies predominantly on circulating glucose to meet its energy needs.

Given Data / Assumptions:

• Under normal dietary conditions, neurons use glucose delivered via the bloodstream.
• During prolonged fasting, the brain adapts to use ketone bodies, but this is not the default state.
• Red blood cells rely exclusively on glucose due to lack of mitochondria, but RBCs are not listed as the main stem answer in this set of options.

Concept / Approach:
Compare fuel preferences of tissues. Skeletal muscle can use fatty acids and glucose; kidney cortex uses fatty acids, while medulla uses glucose; liver prefers fatty acids and does not consume much glucose for energy. The brain is classically taught to be glucose-dependent (with ketone utilization only after adaptation), making it the best choice among the provided options.

Step-by-Step Solution:

Verification / Alternative check:
During hypoglycemia, neuroglycopenic symptoms develop rapidly, highlighting the brain’s acute reliance on glucose supply in the absence of adapted ketosis.

Why Other Options Are Wrong:

Common Pitfalls:
Overlooking the brain’s ability to adapt to ketone bodies after several days of fasting; confusing baseline preference with starvation adaptation.

Final Answer:
Brain cells (neurons and glia).