B12 Deficiency in Infants: Pseudo-TMA and USMLE High-Yield

A case report in BMJ Case Reports describes an infant with neurodevelopmental regression and a "pseudo-thrombotic microangiopathy" picture, both caused by vitamin B12 deficiency. The authors call this combination rare. The report is a reminder that a severe neurologic and hematologic presentation in a child can have a correctable cause. It is a single case and says nothing about how often this occurs.

What happened

According to the available summary, the infant presented with two problems at once. The first was neuroregression, meaning a loss of previously acquired developmental skills. The second was a hematologic picture that resembled thrombotic microangiopathy (TMA) but was not a true primary TMA. The authors label it "pseudo-TMA." Both were attributed to vitamin B12 (cobalamin) deficiency.

The summary I have is brief. It does not give the infant's age, the laboratory values, the reason for the deficiency, the treatment, or the outcome. I won't guess at those details here. For the full clinical course, read the original report in BMJ Case Reports.

Why it matters

TMA is a medical emergency. Hemolytic anemia with thrombocytopenia and schistocytes raises concern for TTP, HUS, or other TMA syndromes, and those diagnoses lead to very different management from a nutritional deficiency. When neurologic regression is added, the differential widens further to include metabolic, infectious, and genetic disorders.

The teaching point is that B12 deficiency can imitate a TMA, so it belongs on the differential in an infant with this combination. The cause is treatable, and recognizing it matters because the underlying problem is metabolic rather than a primary microvascular disease.

What to know for the USMLE

This topic ties together biochemistry, hematology, neurology, and pediatrics. These are the textbook points that matter for Step 1 and Step 2 CK.

• Two B12-dependent reactions.
• Methionine synthase (methylcobalamin) converts homocysteine to methionine and uses methyl-THF as the methyl donor.
• Methylmalonyl-CoA mutase (adenosylcobalamin) converts methylmalonyl-CoA to succinyl-CoA.
• Lab signature. B12 deficiency raises both homocysteine and methylmalonic acid (MMA). Folate deficiency raises homocysteine but leaves MMA normal. This is a classic exam discriminator.
• Why anemia develops. Impaired methionine synthase traps folate as methyl-THF (the "folate trap"). This limits thymidine and DNA synthesis, producing megaloblastic anemia with macrocytosis, hypersegmented neutrophils, and pancytopenia in severe cases.
• Why a TMA-like picture can occur. Megaloblastic erythropoiesis is ineffective, and many precursors die in the marrow (intramedullary hemolysis). This can produce high LDH, high indirect bilirubin, low haptoglobin, and sometimes fragmented red cells and low platelets. These findings overlap with TMA. This is textbook background and not a statement about the reported infant's labs.
• Why the nervous system is affected. The mechanism is incompletely understood. The classic teaching is that the MMA/propionate pathway is disrupted, leading to abnormal myelin and demyelination. In adults this appears as subacute combined degeneration (dorsal columns and lateral corticospinal tracts). In infants it can appear as hypotonia, irritability, feeding difficulty, developmental delay or regression, and sometimes seizures.
• Causes in infants (textbook list).
• Maternal B12 deficiency, for example from a strict vegan diet or maternal pernicious anemia, especially in an exclusively breastfed infant.
• Malabsorption, such as Imerslund-Gräsbeck syndrome.
• Transcobalamin deficiency.
• Inborn errors of intracellular cobalamin metabolism, such as cblC disease.

The summary does not say which applied here.

• Mimics to separate from true TMA.
• TTP shows severely reduced ADAMTS13 activity.
• Typical HUS follows Shiga toxin–producing E. coli.
• Complement-mediated (atypical) HUS is another consideration.
• Combined methylmalonic aciduria with homocystinuria (cblC) can itself present with TMA features.
• In true TMA, the Coombs test is negative and PT/aPTT are typically normal. These findings alone do not exclude B12 deficiency.
• Pharmacology pearl. B12 deficiency is treated with parenteral or high-dose cobalamin replacement. Giving folate alone can improve the anemia while the neurologic damage progresses. This is a classic exam trap.
• Step 2 CK reasoning. If a child has regression plus unexplained hemolysis or cytopenias, check B12, folate, MMA, homocysteine, a peripheral smear, and reticulocyte count along with the TMA work-up. Use the clinical context and local protocols to decide the order and urgency.

Limitations

• This is a single case report. It cannot show how common this presentation is, and the authors' description of it as rare is not a measured frequency.
• No causal generalization. One patient cannot establish that B12 deficiency causes pseudo-TMA across infants, or define its risk factors.
• Limited source detail. I worked from a short summary, not the full text. Laboratory data, the cause of deficiency, treatment response, and long-term neurologic outcome are not covered here.
• No treatment guidance. This article is educational and does not address the care of any individual patient.

FAQ

Can vitamin B12 deficiency really mimic thrombotic microangiopathy?

According to this case report, yes, in a rare presentation the authors describe as pseudo-TMA. Textbook physiology offers a plausible explanation: ineffective erythropoiesis and intramedullary hemolysis can produce TMA-like laboratory findings. One case does not show how often it happens.

How do MMA and homocysteine help in diagnosing B12 deficiency?

Both rise in B12 deficiency because both B12-dependent enzymes are impaired. In folate deficiency, homocysteine rises but MMA stays normal. This is a high-yield pattern for both Step 1 and Step 2 CK.

Why is B12 deficiency dangerous for infant brain development?

Cobalamin is needed for myelin integrity and for normal methylation and amino acid metabolism. Deficiency can cause hypotonia, developmental delay, or regression. Exam questions often pair these findings with a breastfed infant of a mother with pernicious anemia or a strict vegan diet.

Key takeaways

• A BMJ Case Reports report links infant B12 deficiency to neuroregression and a pseudo-TMA picture, a combination the authors call rare. It is a single case and cannot support conclusions about frequency.
• For the USMLE, remember elevated MMA and homocysteine, megaloblastic anemia, neurologic involvement, and the folate-trap mechanism. Remember too that folate alone does not protect the nervous system.
• A TMA-like picture does not always mean primary TMA. Metabolic and nutritional causes belong on the differential, especially in an infant with developmental regression.