Correct Answer: D. Intracellular fluid
The electrolyte composition provided—Na 10 mEq/L, K 140 mEq/L, Cl 4 mEq/L—is pathognomonic for intracellular fluid (ICF). The discriminating feature is the inverse Na:K ratio: high potassium and very low sodium. This reflects the action of the Na⁺-K⁺-ATPase pump, which actively extrudes 3 Na⁺ ions outward and pumps 2 K⁺ ions inward, maintaining the steep electrochemical gradient essential for cellular function. In ICF, K⁺ is the dominant cation (140 mEq/L), while Na⁺ is kept low (10 mEq/L) to preserve membrane potential and enable excitability. Chloride is minimal (~4 mEq/L) because the negatively charged intracellular proteins (phosphates, sulfates, organic acids) serve as the major anions. This composition is consistent across all body cells—muscle, nerve, liver, kidney—and is fundamental to cellular physiology as taught in Guyton's Physiology and Harrison's Principles. The clinical relevance in Indian practice: hyperkalemia (K⁺ >5.5 mEq/L) or hypokalemia (K⁺ <3.5 mEq/L) reflects ICF-ECF shifts and is a critical emergency in acute care settings.
Why the other options are wrong
A. Interstitial — Interstitial fluid is part of the extracellular compartment and has an electrolyte composition nearly identical to plasma: Na⁺ 140 mEq/L, K⁺ 5 mEq/L, Cl⁻ 104 mEq/L. The given sample shows K⁺ 140 and Na⁺ 10, which is the opposite pattern. This is a common trap—students may confuse 'body fluid' with any non-plasma fluid, but interstitial fluid maintains ECF composition. **B. Extracellular fluid** — ECF (plasma + interstitial) is characterized by high Na⁺ (140 mEq/L), low K⁺ (5 mEq/L), and high Cl⁻ (104 mEq/L)—the opposite of the given values. NBE may trap students who remember 'ECF is the internal environment' but forget the specific cation ratios. The sample clearly shows ICF dominance of K⁺. C. Plasma — Plasma is the liquid portion of ECF with Na⁺ 140 mEq/L, K⁺ 5 mEq/L, Cl⁻ 102 mEq/L, and contains proteins (7 g/dL). The given sample's K⁺ of 140 and Na⁺ of 10 are completely inconsistent with plasma. This option tests whether students can distinguish plasma from ICF by electrolyte pattern alone.
High-Yield Facts
- ICF Na⁺:K⁺ ratio is ~1:14 (10:140 mEq/L), maintained by Na⁺-K⁺-ATPase; ECF ratio is ~28:1 (140:5 mEq/L)—opposite pattern is the key discriminator.
- Intracellular anions are organic phosphates, sulfates, and proteins (150 mEq/L), not chloride; Cl⁻ is minimal (4 mEq/L) because it cannot cross the cell membrane easily.
- Na⁺-K⁺-ATPase uses 20–40% of resting cellular ATP to maintain the gradient; dysfunction causes cell swelling and loss of excitability.
- ICF comprises ~40% of body weight (27 L in 70 kg adult); ECF is ~20% (14 L)—the larger compartment has opposite electrolyte composition.
- Hyperkalemia (K⁺ >5.5 mEq/L) in plasma signals ICF-to-ECF K⁺ shift, seen in rhabdomyolysis, tumor lysis, acidosis—critical emergency in Indian ICUs.
Mnemonics
ICF vs ECF Cations ICF: K-IN, Na-OUT (K⁺ high, Na⁺ low inside). ECF: Na-IN, K-OUT (Na⁺ high, K⁺ low outside). The pump does 3 Na⁺ out, 2 K⁺ in—remember 'pump pushes Na out, pulls K in.' The 140-10 Rule If you see K⁺ 140 and Na⁺ 10 in a body fluid, it's ICF. If you see Na⁺ 140 and K⁺ 5, it's ECF/plasma. This is the fastest discriminator on the exam.
NBE Trap
NBE pairs high K⁺ with 'body fluid sample' to trap students who confuse interstitial fluid (which looks like plasma) with ICF. The trap is the word 'sample'—students may assume any non-plasma fluid is interstitial, forgetting that only ICF has this K⁺-dominant composition.
Clinical Pearl
In Indian emergency departments, a patient with rhabdomyolysis or tumor lysis syndrome presents with plasma K⁺ >6 mEq/L because massive ICF K⁺ leaks into ECF. Understanding this ICF-ECF gradient is why we give calcium gluconate, insulin-dextrose, and sodium bicarbonate—to shift K⁺ back into cells and prevent fatal arrhythmias.
_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 25 (Body Fluid Compartments); Harrison's Principles of Internal Medicine, Ch. 46 (Hyponatremia and Hypernatremia)_