Which of the following is a non-essential amino acid?

Appeared in: SGPGI Nursing Officer-2024

Explanation

Cysteine is classified as a non-essential or, more precisely, a conditionally essential amino acid.
The human body can synthesize Cysteine using other molecules as precursors, specifically the essential amino acid Methionine and the non-essential amino acid Serine.
Because the body has a pathway to produce it, Cysteine is not required to be present in the diet under normal, healthy conditions, unlike essential amino acids.

Option B: Leucine is an essential amino acid, meaning the body cannot synthesize it, and it must be obtained from dietary sources.
Option C: Histidine is an essential amino acid that must be supplied through the diet, especially for infants and children for proper growth.
Option D: Methionine is an essential, sulfur-containing amino acid that cannot be produced by the body and is required for various metabolic processes, including the synthesis of Cysteine.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Classification of amino acids (essential vs. non-essential) as background academic context rather than a clinical decision trigger.
Understanding the classification of amino acids is crucial for nutritional assessment and planning, especially for patients with malnutrition, liver disease, or those requiring parenteral nutrition (PN).
In clinical practice, certain 'non-essential' amino acids like Cysteine and Arginine become 'conditionally essential' during periods of metabolic stress such as trauma, sepsis, or major surgery, requiring supplementation.
What if? If a patient has a genetic disorder preventing the conversion of Methionine to Cysteine, Cysteine would become an essential amino acid for that individual, requiring direct dietary intake to prevent deficiency.

How to Approach the Question

First, understand the question is asking to identify the 'non-essential' amino acid among the options.
Recall the definition: Non-essential amino acids can be synthesized by the body, while essential amino acids must be obtained from the diet.
Try to remember the list of essential amino acids, possibly using a mnemonic like 'PVT TIM HALL' (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine).
Evaluate each option against this list.
Leucine, Histidine, and Methionine are all part of the mnemonic and are therefore essential.
Cysteine is not on the primary list of essential amino acids, indicating it is non-essential (or conditionally essential), making it the correct answer.

Concept Tested & Keywords

Q3-MGqxVRMjx1-PzU4T5eh

E6 Biochemistry U Satyanarayana— Part 1 (pp 26-420 of 840) p. 37-39

E6 Medicine Davidson Principles Practice 24e p. 717-719

E6 Pharmacology Nursing Lilley 11e Part 3 p. 231-233

Practise the full SGPGI Nursing Officer-2024

Attempt every question from this paper in a timed mock, then review the full solution for each one.