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The USMLE will sometimes ask tricky questions where the distinction between these three conditions may seem equivocal. So I’ll highlight the highest yield points:
Marasmus is total-calorie deficiency presenting as diffuse wasting.
In contrast, kwashiorkor is an overwhelming protein malnutrition. Even if total calorie intake is inadequate (it often is), of the calories that are consumed, the lack of protein (e.g., from nothing but ongoing rice consumption), the result is hypoalbuminemia, decreased intravascular oncotic pressure, and, ultimately, ascites.
So far so good. Not hard.
Now we introduce cretinism, which is congenital hypothyroidism.
The USMLE will try to trick you because cretinism can also present with protruding abdomen.
| On the USMLE, cretinism will always present with severe mental impairment, whereas kwashiorkor frequently will not. Cretinism will also frequently present with large tongue and impaired linear bone growth. This is because thyroid hormone is necessary for myelin and skeletal development. |
It should also be noted that, worldwide, the most common cause of hypothyroidism is iodine deficiency. But in western countries, the most common cause is Hashimoto thyroiditis.
Some USMLE distinctions:
| Protruding abdomen alone mentioned.
Is there any mental impairment? No. → Kwashiokor Is there any mental impairment? Yes, there’s mental impairment. → Cretinism |
If the question is relatively vague but presents what seems like a malnourished infant, if they go out of their way to mention that the bones aren’t developing well, cretinism is the answer because thyroid hormone is necessary for bone development, as we’ve mentioned.
Also, if the vignette gives you a patient with kwashiorkor and asks for the amino acid that will best treat it, the answer usually leucine, lysine, or tryptophan.
Technically there are nine essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) and six conditionally essential amino acids (arginine, cysteine, glutamine, glycine, proline, tyrosine). You do not need to memorize these for the USMLE.
So why lysine or tryptophan? Yeah I know, weird. But let’s just run with it. It’s what the USMLE wants and they’ve shown up in questions as the answer.
| The heel-prick test done at birth, where a drop of blood is taken from the neonate to be screened for congenital hypothyroidism, PKU, and galactose disorders, among others, is performed notably to detect as early as possible preventable causes of mental retardation. |
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