A man from an endemic region has developed progressive leg swelling. Peripheral smear reveals the following parasitic organism. Which pathophysiologic mechanism is most likely responsible?

Correct Answer: B. Lymphatic obstruction

The clinical presentation of progressive leg swelling in an endemic region with parasitic organisms on peripheral smear is pathognomonic for lymphatic filariasis (caused by Wuchereria bancrofti, Brugia malayi, or Brugia timori). The adult worms reside in lymph nodes and lymphatic vessels, causing chronic inflammation, fibrosis, and progressive obstruction of lymphatic channels. This mechanical blockade of lymphatic drainage is the PRIMARY pathophysiologic mechanism leading to lymphedema and elephantiasis. The obstruction prevents normal lymph flow from distal tissues, causing fluid accumulation in the interstitial space. While secondary hypoalbuminemia and hypoproteinemia may develop due to protein-losing enteropathy or malnutrition in chronic filariasis, these are NOT the immediate or primary mechanism of leg swelling. The swelling occurs acutely when lymphatic vessels are obstructed, independent of serum protein levels. Increased hydrostatic pressure is a feature of venous obstruction or cardiac failure, not lymphatic filariasis. In India, W. bancrofti is endemic in coastal and deltaic regions (West Bengal, Odisha, Tamil Nadu), making this a high-yield clinical scenario for NEET PG.

Why the other options are wrong

A. Hypoalbuminemia — While chronic filariasis may cause secondary hypoalbuminemia through protein-losing enteropathy or malnutrition, this is NOT the primary mechanism of acute lymphedema. Hypoalbuminemia develops slowly and is a consequence of chronic disease, not the direct cause of leg swelling. The swelling occurs immediately upon lymphatic obstruction, independent of serum albumin levels. This is an NBE trap that confuses secondary effects with primary pathophysiology. C. Hypoproteinemia — Similar to hypoalbuminemia, hypoproteinemia is a late complication of chronic filariasis, not the primary mechanism. The lymphedema develops due to mechanical obstruction of lymph flow, which occurs regardless of total protein levels. Confusing this secondary nutritional consequence with the direct pathophysiologic mechanism is a common NEET trap that tests understanding of cause vs. effect. D. Increased hydrostatic pressure — Increased hydrostatic pressure causes edema in venous obstruction (DVT, cardiac failure, cirrhosis) or in conditions with elevated capillary pressure. Lymphatic filariasis causes edema through DECREASED lymphatic drainage (obstruction), not increased hydrostatic pressure. The worms obstruct lymph vessels, reducing fluid reabsorption—the opposite mechanism. This option tests whether students confuse lymphatic vs. venous/cardiac pathophysiology.

High-Yield Facts

Mnemonics

FILARIA Pathophysiology Fibrosis of lymphatics → Inflammation → Lymphatic Arrest → Retention of fluid → Increased A (edema). Remember: the worm BLOCKS the pipe, not the protein. Endemic Filariasis in India (COAST) Coastal regions, Odisha, Andhra Pradesh, South India (Tamil Nadu), Tamil Nadu. W. bancrofti loves deltaic and coastal zones.

NBE Trap

NBE pairs filariasis with hypoalbuminemia/hypoproteinemia to test whether students confuse secondary nutritional complications with the PRIMARY pathophysiologic mechanism. The key discriminator is that lymphedema develops acutely upon worm-induced lymphatic obstruction, independent of serum protein levels.

Clinical Pearl

In Indian clinical practice, a patient from West Bengal or Odisha with progressive unilateral leg swelling and microfilariae on blood smear is filariasis until proven otherwise. The swelling worsens acutely during episodes of lymphangitis (when adult worms die or trigger immune response), confirming that mechanical obstruction—not protein deficiency—is the driver. Early DEC therapy can halt progression if given before irreversible fibrosis occurs.

_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology Ch. 46 (Filarial Nematodes); Park's Textbook of Preventive and Social Medicine Ch. 7 (Filariasis); Harrison's Principles of Internal Medicine Ch. 219 (Helminthic Infections)_