**Core Concept**
Severe malnutrition is diagnosed based on serum albumin levels, which reflect the body's nutritional status and albumin synthesis capacity. Serum albumin is a protein synthesized by the liver, with a half-life of approximately 20 days. Low serum albumin levels indicate inadequate nutrition, liver disease, or other conditions affecting protein synthesis.

**Why the Correct Answer is Right**
The correct answer is based on the World Health Organization's (WHO) criteria for malnutrition, which defines severe malnutrition as a serum albumin level less than **30 g/L**. This threshold is used to identify patients at risk of complications and mortality due to malnutrition. The liver produces albumin, which is essential for maintaining oncotic pressure and transporting various substances in the bloodstream. Low serum albumin levels indicate impaired liver function or inadequate nutrition, leading to complications such as edema, ascites, and impaired wound healing.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum albumin levels below 20 g/L are not typically considered severe malnutrition, as they may be seen in various conditions, including liver disease, nephrotic syndrome, or chronic diseases.

**Option B:** Serum albumin levels between 20-30 g/L are often considered moderate malnutrition, which may not require the same level of medical intervention as severe malnutrition.

**Option C:** Serum albumin levels above 40 g/L are generally considered normal and do not indicate malnutrition.

**Clinical Pearl / High-Yield Fact**
A serum albumin level of less than 30 g/L is a strong predictor of mortality and complications in hospitalized patients, especially those with liver disease, cancer, or chronic infections.

**Correct Answer: C. 35 g/L**