**Core Concept**
The question is testing the knowledge of congenital tuberculosis (TB), a rare but serious condition where TB is transmitted from a pregnant woman to her fetus. The most common organ involved in congenital TB is related to the pathophysiology of TB transmission and the anatomical development of the fetus.

**Why the Correct Answer is Right**
Congenital TB typically affects organs that are in close proximity to the placenta and are involved in the maternal-fetal interface. The most common organ involved in congenital TB is the **liver**, which receives a high volume of maternal-fetal blood flow. The liver is also a common site for TB infection due to its rich blood supply and the presence of Kupffer cells, which are phagocytic cells that can ingest TB bacilli. The liver's involvement in congenital TB is often related to the hematogenous spread of TB bacilli from the maternal bloodstream to the fetal liver.

**Why Each Wrong Option is Incorrect**
* **Option A:** The lungs are not the most common organ involved in congenital TB, although they can be affected in some cases. The lungs are a common site for pulmonary TB, but congenital TB typically involves organs that are not yet fully developed during fetal life.
* **Option B:** The spleen is not typically involved in congenital TB, although it can be affected in cases of disseminated TB. The spleen's role in filtering the blood and storing red blood cells makes it more susceptible to TB infection, but it is not the most common organ involved in congenital TB.
* **Option C:** The kidneys are not typically involved in congenital TB, although they can be affected in cases of disseminated TB. The kidneys' role in filtering waste and excess fluids makes them more susceptible to TB infection, but they are not the most common organ involved in congenital TB.

**Clinical Pearl / High-Yield Fact**
The key to diagnosing congenital TB is to suspect it in any newborn with unexplained respiratory distress, failure to thrive, or hepatosplenomegaly. The diagnosis of congenital TB is often challenging and requires a high index of suspicion, as the clinical presentation can be nonspecific and the diagnosis is often made post-mortem.

**Correct Answer: C. Liver**