Correct Answer: B. Breslow index
Breslow index is a thickness measurement of subcutaneous fat used in melanoma staging and prognosis assessment, not for general body fat assessment. It measures the vertical thickness of a melanoma lesion in millimeters and is critical for determining melanoma depth and TNM staging. In contrast, Quetelet's index (BMI = weight/height²), total body potassium (TBK), and total body water (TBW) are all validated methods for assessing total body fat composition. Quetelet's index is the most widely used anthropometric screening tool in Indian clinical practice and epidemiological surveys. TBK and TBW are indirect methods based on the principle that fat-free mass contains a constant proportion of potassium and water; these are used in research and clinical settings where precise body composition analysis is needed. The Breslow index's exclusive application to melanoma depth measurement makes it fundamentally different from the other three options, which are specifically designed for body fat quantification across populations.
Why the other options are wrong
A. Quetelet's index — Quetelet's index (BMI = weight in kg / height in m²) is the gold standard anthropometric screening tool for assessing body fat and nutritional status in Indian population surveys and clinical practice. It is universally used in NRHM, ICDS, and hospital nutrition assessments. This is a direct method for body fat assessment, making it a correct answer to the question. C. Total body potassium — Total body potassium (TBK) is an indirect method for assessing body composition because potassium is found almost exclusively in fat-free mass (muscle, organs, bone). By measuring TBK using whole-body counting, clinicians can calculate fat-free mass and derive total body fat. This is a validated research and clinical method, particularly in metabolic and nutritional assessment. D. Total body water — Total body water (TBW) is used to estimate fat-free mass because water comprises approximately 73% of fat-free mass. Measured via isotope dilution (deuterium or tritium), TBW allows calculation of fat mass by subtraction. This is a reference standard method in body composition research and clinical nutrition assessment.
High-Yield Facts
- Quetelet's index (BMI) = weight (kg) / height² (m²); normal range 18.5–24.9 kg/m² in Indian adults; primary screening tool for undernutrition and overweight.
- Breslow index measures melanoma thickness in millimeters; used for TNM staging and prognosis, NOT body fat assessment.
- Total body potassium (TBK) is an indirect body composition method; assumes constant K⁺ content in fat-free mass (~60 mmol/kg).
- Total body water (TBW) estimated via isotope dilution; fat-free mass = TBW / 0.73; reference standard for body composition research.
- Body fat assessment methods in India: anthropometry (BMI, skinfold thickness), bioelectrical impedance analysis (BIA), and indirect calorimetry in research settings.
Mnemonics
BODY FAT METHODS (QTW) Quetelet's index, Total body potassium, Total body water — all assess body fat. Breslow is for melanoma thickness (Skin cancer, not skin fat). BRESLOW = MELANOMA DEPTH Breslow measures tumor thickness in mm; used for staging melanoma (AJCC TNM), not for assessing patient body composition or nutritional status.
NBE Trap
NBE exploits the fact that students may confuse Breslow index with other anthropometric/body composition indices. The trap is that Breslow sounds like a measurement scale (like Broca's index for height-weight), but it is actually a melanoma-specific pathological measurement, not a nutritional assessment tool.
Clinical Pearl
In Indian ICDS and NRHM screening programs, Quetelet's index (BMI) is the only practical tool used at the grassroots level for identifying malnutrition and overweight in children and adults. Breslow index is encountered only in oncology settings when staging melanoma — a rare malignancy in Indian populations with darker skin, making this distinction clinically important for exam preparation.
_Reference: Park's Textbook of Preventive and Social Medicine (Ch. Nutrition and Health); Harrison's Principles of Internal Medicine (Ch. Melanoma and Skin Cancers)_